UC BERKELEY MASTER NEGATIVE STORAGE NUMBER 03-67.33 (National version of master negative storage number: CU SN03067.33) MICROFILMED 2003 UNIVERSITY OF CALIFORNIA AT BERKELEY LIBRARY PHOTOGRAPHIC SERVICE REPRODUCTION AVAILABLE THROUGH INTERLIBRARY LOAN OFFICE MAIN LIBRARY UNIVERSITY OF CALIFORNIA BERKELEY, CA 94720-6000 COPYRIGHT The copyright law of the United States (Title 17, United States Code) governs the making of photocopies or other reproductions of copyrighted materials including foreign works under certain conditions. In addition, the United States extends protection to foreign works by means of various international conventions, bilateral agreements, and proclamations. Under certain conditions specified in the law, libraries and archives are authorized to furnish a photocopy or other reproduction. One of these specified conditions is that the photocopy or reproduction is not to be "used for any purpose other than private study, scholarship, or research." If a user makes a request for, or later uses, a photocopy or reproduction for purposes in excess of "fair use," that user may be liable for copyright infringement. University of California at Berkeley reserves the right to refuse to accept a copying order if, in its judgment, fulfillment of the order would involve violation of copyright law. Koch, Thomas W. Geological relations on the Tiburon Peninsula 1924 BIBLIOGRAPHIC RECORD TARGET University of California at Berkeley Library Master negative storage number: 03-67.33 (national version of the master negative storage number: CU SN03067.33) GLADIS NUMBER: 1847872410 FORMAT : BK AD:991009/FZB LEVEL:b BLT:am DCF:a CSC:d MOD: EL:"7 UD:030604 /MAP CP:cau L:eng INT: GPC: BIO: FIC: CON: ARCV: PC:s PD:1924/ REP: CPI: FSI: ILC: ITI:0 CUScCU SbDISS.KOCH.GEOL 1924 Koch, Thomas W. Geological relations on the Tiburon peninsula. Scl924. 32 p. :$bill., col map ;S$c29 cm. Thesis (B.S. in Geology )-- University of California, Berkeley, 1924. 20 University of California, Berkeley.$bDept. of Geology and GeophysicsSxDissertations. 0 Dissertations, Academic$xUCB$xGeologyS$y1921-1930 Microfilmed by University of California Library Photographic Service, Berkeley, CA FILMED AND PROCESSED BY LIBRARY PHOTOGRAPHIC SERVICE, UNIVERSITY OF CALIFORNIA, BERKELEY, 94720 DATE: 7/03 REDUCTION: 10 X PM-1 3%"x4" PHOTOGRAPHIC MICROCOPY TARGET NBS 1010a ANSI/ISO #2 EQUIVALENT 10 “hz J ve [132 pa ew BE TE ll ££," l= be LZ lis ie me 1M NY aly 1g et v1. elt alt, tt, ot, |6, [8 eu et at Ut oo, 6 18 | 9 | Sg |P HIN GEOLOGICAL RELATIONS ON THE TIBURON PENINSULA. LIBRARY COPY Thomas W. Koch. -CORTENTS Page Purpose Acknowledgments Location PHYSICAL FEATURES GEOLOGICAL FEATURES Lithology of Normal Types GENERAL STATEMENT THE GREENSTONE TS SS — — G — "WTP YR > GD GW TG Sq — THE SERPENTINE --- a — Structure & Age Relations THE TIBURON SYNCLINE -- THE OCCURRENCE OF THE GREENSTONE -- THE OCCURRENCE OF THE SERPENTINE - The Metamorphic Rocks GENERAL RELATIONSHIPS 2 METAMORPHOSED CHERT -——— — a 1} METAMORPHOSED SANDSTONE --- METAMORPHOSED GREENSTONE CONCLUSIONS - APPENDIX Bibliography Map - Cross Section tn Plates --=w=e=- GEOLOGICAL RELATIONS ON THE TIBURON PENINSULA. INTRODUCTION Purpose This paper was written in partial fulfillment of the requirements for the degree of Bachelor of Science in the College of Mining, University of California. It is hoped, also, that the reader will gain some insight into the geologic relationships and phenomena exhibited in this classic locality which have not been set forth by previous investigators. Acknowledgment s The writer wishes to express his most grateful appreciation to Dr. Roy R. Morse under whose supervision this work was carried on,and to Mr. Norman Hardy with whom the work was done and whose efforts have done much in making possible this report. Location The locality to which this report refers includes the south-eastern end of the Tiburon Peninsula which, as a whole, extends for a distance of about five miles into San Francisco Bay. It lies seven miles north of San Francisco and directly across Richardson's Bay from Sausalito. The area mapped includes roughly three scuare miles. The peninsuls is about a mile wide. PHYSICAL FEATURES The Tiburon Peninsula is elongated in approximately a north-west and south-east direction, which is also the general trend of the serpentine body which may be thought of as forming the back-bone of the peninsula. The north- eastern slopes are noticeably steeper then those on the south-western side,and are covered with a light growth of scrub osk smd brush. The south-western slopes, on the other hand, are remarkably bare. Outcrops are abundant, but rather varied and sporadic in the contact sureole surrounding the central serpentine mass. Between such outcrops is a soil mantle which gives no evid- ence of the underlying rock types. In other parts than in the contact zone, however, the soil is characteristic of the underlying rocks. GEOLOGICAL FEATURES Lithology of Normal Types GENERAL STATEMENT The rocks found on the Tiburon Peninsula comprise a group made up wholly of types referred to the Franciscan (1) series. The Franciscan is found along the Pacific Coast from San Pedro northward into Oregon,and possibly far beyond. It is composed in large part of marine cherts with interbedded shale, sandstones having shaly and conglomeratic phases, and certain basic igneous rocks occurring as hypabyssal and surface bodies. The age of the series has been placed by various writers (1), (2), (3), (4), in the Cretaceous, the Jurassic, and the Triassic. It may well cover all three of these periods. THE RADIOLARIAN CHERT FORMATION. ‘ The chert of the Franciscan of Tiburon is commonly of a reddish to buff- colored or a greenish hue. It occurs seldom massive but habitually is sharply laminated with narrow buff colored siliceous shale bands. The chert and shale laminae vary in thickness from several feet to a fraction of an inch, the most common thickness being about an inch. Gradation by an increase or decrease in the thickness of shale partings to pure chert or pure shale is not uncommon. The cause for the exceedingly regular lamination of chert and shale in the Franciscan has been discussed by Davis (5) and others. In the field the chert gives either a much cremmlated and contorted out- crop which is ordinarily light colored,or else it is observable only as a reddish-brown hackly mass covered by little or no soil,but not outstanding above the general surface. A fracture in the rock normal to the bedding which crosses several laminae, gives, in those beds standing at high angles, a characteristic flat topped outcrop. (Plate 1) On the cleavage and fracture planes dendrite is common. Manganese occurs also in the form of large and small ore bodies, clearly interbedded in the cherts. (6) Microscopically, the radiolarian chert is usually dense and aphanatic although some specimens may be collected in which the tiny radiolarian skeletons giving it its name may be distinguished. Under the microscope, however, the chert appears as a mass of cryptocrystalline silica in which there are many round spots of more transparent silica, the remnants of the tests of the radio- laria. (Plate V11l) Secondary silica occurs abundantly as veins of gramular quartz throughout the formation. Disintegration of the chert member gives a reddish-brown shallow soil filled with chert rhombs. Davis (5 & 8), Smith (9) and others (7) recognize more than one chert member in the Franciscan with interbedded sandstones. All, however, are similar lithologically. One major chert member only has been recognized at Tiburon although local lenses occur within the sandstone. (See cross-section). THE SANDSTONE FORMATION. The sandstone of the Franciscan series here is a typical arkose. Its principal constituents in the order of their relative abundance are quartz, feldspar, mica and rock fragments. It grades in some places into a shaly facieszand at times lenses of conglomerate occur, containing boulders having a maximum diameter of one foot, embedded in a matrix of normal sandstone. (Sta. 17). When fresh, the sendstone is a hard, bluish-grey rock, but it easily weathers to a brown, crumbly mass of sand grains. It is very massive and in such few outcrops as occur, presents an exceptionally uniform even grained appearance. The equigramular character of the grains is also apparent in the hand specimen together with the extreme freshness and angularity of the feld- spars. The plagioclases are about of the composition of oligoclase. Davis (5) also finds orthoclase. The quartz is little rounded in general, but some grains show considerable wear, These were probably derived from older sediments. Both muscovite and biotite are common. The arkosic nature of the rock is apparent from these observations. An unusal constituent, but persistent and widespread in this sandstone, especially in the more conglomeratic phases, is angular soft black shale. At times these fragments become so abundant as to make up nearly one-third of the rock. The conglomeratic facies consist, as mentioned above, of boulders in a matrix of normal sandstone. Except for the presence of the angular shale flakes, the boulders are well rounded, much in contrast to the angular constituents of the sandstone matrix. The boulders comprise such types as hard quartzites and igneous rocks of about the composition of diorite, diabase,and basalt. In an outcrop to the north of the area mapped, pebbles of radiolarian chert and a green- stone similar to the one found in this area were discovered. These conglomerates in the sandstone occur as minor lenses and major members of the formation at no definite horizon. They are not limited to the basal portions of the formation and they show rapid gradations into normal sandstone. Davis (5), as mentioned in discussing the cherts, finds, in the Franciscan interbedded chert and sandstone members. The various sandstone horizons, similar lithologically, have been called the Cahil, the Marin, and the Bonita. Although there is possibility of the presence of two sandstone members at Tiburon, but one sandstone has been assumed at the southern end. Secondary veins of calcite, quart z and albite are found in the sandstone. THE GREENSTONE. The greenstone member appears in the field, either as a bare, crumbly brown mass similar to the weathered chert and not outstanding above the ground surface; or it may be somewhat more outstanding giving the rounded, hackly appearance of some serpentine boulders; or lastly, in a more distinctive form seen especially on the southern shore of the peninsula, as rounded, holy,” pillow shaped masses (Plate V) comparable to the ellipsoidal lavas of Capps. (10) Upon procuring a fresh specimen of the rock it is seen to be very dense, hard, and fine-grained, with a dark green to almost black color. In some specimens, lath-shaped feldspars are distinguishable in the felsitiec roma mass with the hand lens,and at Sta. 28 phenocrysts of feldspar showing equant development one-half inch in diameter were found. Just north of Sta. 28 an amygdaloidal facies of the greenstone was observed which could be directly traced by a reduction in the number of Cavalieh in- to the dense common type. The gradation took place within two feet. Microscopically, the greenstone is found to have a ground mess of felted andesine laths with, at times, a little glass and interstitial augite now gone over wholly to chlorite. Phenoerysts are of labradorite-bytownite (Abzy Any) with occasional Augite crystals, (Plate 1X). Flow structure is marked in some specimens but totally absent in others. The rock appears to be an Augite Andesite. THE SERPENTINE. A feature very noticeable at Tiburon is the rugged brown and gray-green mass of serpentine, occupying the central portion of the peninsula. (Plate II). It stands above the surrounding formations weathering out as immense rugged boulders or else, in more limited areas, as a foliated light green to grey rock, the planes of foliation being always nearly vertical. In the more massive facies, the appearance is of a boulder conglomerate (Plate III). A fresh specimen of serpentine shows the relic structure of the perid- otite from which it was derived. Platy bastite gives evidence of rhombic pyroxene. Monoclinic pyroxenes were also present according to Ransome (7) who classes this as a derivative from Lherzolite. A microscopic examination of the rock revealed nothing that showed the presence originally of monoclinic pyroxene and it is therefore thought that this is a saxonite. The bastite texture is evident in the slides (Plate X), but the pemdfgurghs after eustate give reactions for chrysotile instead of bastite as might be expected. Enclosing the pseudo” phenocrysts is fibrous serpentine in what appears to be a mesh structure, giving evidence of former olivene, In addition, grains of magnetite are present along cracks and in irregular patches throughout the mass. Suites of specimens taken across the serpentine body and at various points widely separated show no significant variation in texture or composition. Structure and Age Relations THE TIBURON SYNCLINE. The rocks of the Tiburon Peninsula, except the serpentine, have been folded into a syncline whose axis has a general trend approximately parallel to the direction of elongation of the peninsula. The majority of the measured attitudes as well as the general distribution of the various formations, (although these are necessarily somewhat idealized because of the obscurity of the contact relations), give evidence that the synclinal axis strikes about N 750 W and plunges downward to the south at a small angle. (See cross section). Dips on either side of the axis vary on account of the minor flextures and puckers developed as well as the distorting influence of the serpent ine mass, but on the north, they appear to be about 60° westerly, and on the south side of the axis about 40° easterly. The serpentine mass occupies nearly the axial portion of the fold. At the southern end, the axis appears to swing away from the serpentine toward a the east to Bluff Pt. Such an interpretation of structure, however, is not in accord with the supposition, which seems hard to avoid unless faulting has occurred, that Angel Island, lying half a mile away across Raccoon Strait, is simply a continuation of the Tiburon Peninsula. Ransome (7) has found on Angel Island a synclinal structure, a projection of whose axial plane would intersect the Tiburon Peninsula about midway between Blufs Pt. and Pt. Tiburon. If the assumption of a synclinal structure be correct, the chert member occupies the central portion of the fold and is flanked by the older sandstone. The sandstone occurring on the eastern side of the peninsula and dipping southerly may be the correlative of the similar sandstone dipping northerly on Belvedere. (See map). In the light of present knowledge, this correlation seems highly probable. Such conclusion, however, again favors a synclinal axis surging more nearly midway between Bluff Pt. and Pt. Tiburon, and it is, therefore, very probable that the apparent eastward swing of the synclinal axis, above referred to,is simply a minor flexture in the sandstone. A conglomerate referred to above (Page 4) as containing radiolarian chert boulders leads us to the conclusion, if the boulders were derived from the chert exposed here and found to be younger than the sandstone member, that a younger conglomeratic deposit exists. This necessitates more than one sandstone member and is in accord with findings of Davis (5). Minor folds are common in the sediments and occur in size from minute crenulations to puckers of mapable size. A good example is seen at Sta. 8 where the chert passes through a minor anticline from its general dip to the north, and then is turned to an almost vertical attitude at the serpentine contact. (Plate 1V). The ground surface here reflects the underground structure very well. 3Juch up-warping and even overturning of beds is common about the periphery of the serpentine mass. THE OCCURRENCE OF THE GREENSTONE, Of the igneous rocks, the greenstone is obviously the older since it is found as boulders in certain conglomerates cut by the serpentine, and also because of its presence as inclusions within the serpentine at two localities. (See map). The mode of occurrence of the greenstone can not be definitely set forth. Lvidence favoring intrusive sills and dykes as well as surface flows is at hand. Favoring the intrusive origin of the greenstone are the following points: (1) It is not restricted to a single sedimentary horizon. (2) It is not always parallel to the bedding. (3) Outerops are not continuous, but rather characteristically separated, suggesting a series of dykes, sills,and offshoots along lines of weakness. (4) Flow structure is not evident in many rocks. Favoring a flow or series of flows as the origin of the greenstone, these roints may be cited: (1) Amygdaloidal structure has been observed. (2) Flow structure is present in certain rocks. (3) The pillow structure observable on the coast clearly occurs as sub-agueous flows. (Plate V). (4) The greenstone is most often conformable with the bedding. (5) No tearing or mechanical deformation of beds has been aseribed to the action of the greenstone. Although the evidence here set down seems to Lavor, to some extent, a flow origin, nevertheless it seems probable that some of the greenstone at least came in as hypabyssal bodies. The age of the greenstone relative to the sediments in this portion of the point is made clear by the inclusion of chert boulders within it near Pt. Tiburon. (Plate V1). The appearance four miles north, however, of apparently this same greenstone in a conglomerate limits its age, if it be truly the same greenstone, to the time between the deposition of the chert here and the over- lying conglomerate further north. THE OCCURRENCE OF THE SFRPENTINE. The serpentine is obviously the youngest member of the Franciscan at Tiburon since it outs all others deforming the youngest sediment at its contact. Its distribution, aside from the main mass, occupying the synclinal axis, is rather widespread as isolated exposures here and there throughout the mapped area,which may be thought of as offshoots and apophyses from the main masse. Field evidence, except for certain irregularities in outline, points toward a dyke as tes mode of occurrence. Its attitude is nearly vertical, departing but slightly to dip steeply to the south-west. The contact often follows straight lines for some distance regardless of topography or underlying structure. It is obviously intrusive from the deformation which adjoining strata have suffered. Its shape in surface outcrop, being about six times as long as it is wide, is indicative of a dyke. Lastly, metamorphism is consider- ably more pronounced on the south-west than on the north-east side of the body, indicating a body dipping south-westerly. Ransome (7) has also mapped the serpentine of Angel Island, which apparently is a continuation of this mass,interrupted by Raccoon Strait, as a steeply dipping dyke. The Metamorphic Rocks GENERAL RELATIONSHIPS. The metamorphic rocks of the Tiburon reninsula. in particular, and of the Franciscan series in general ,have caused considerable discussion on account of their peculiar, highly sodic composition,coupled with their associations and relations with ultra-basic intrusive rocks. The general distribution of metamorphic rocks in this vicinity is far from regular or even contimous for any distance. Rather,these rocks, whose out- crops are nearly always hard and resistant to weathering,will occur in sporadic, disconnected masses undoubtedly in place, but surrounded om all sides by soft soil derived largely from unmetamorphosed material. Such boulders seem, also, to have no relation whatever to each other, either mineralogically or struct- urally. It is, therefore, impossible to trace by gradations, an unmetamorphosed rock into its metamorphosed equivalent along its strike. Metamorphism has affected the same original material to different degrees in spots apparently not genetically related to 1 the intrusive mass. Thus, in one locality, chert beds striking into the serpentine are only slightly recrystallized at the contact, but transformed to a blue schist not over 20 feet away. At other places the chert may be seem to have been completely metamorphosed right at the contact. Again, metamorphosed chert is found at considerable distances from the intrusive with other material between. Similar instances could be cited for the other rock types. The metamorphic rocks may broadly be said to occur within an aureole not over a quarter of a mile wide about the serpentine dyke. This area, however, is not restricted to metamorphic rocks alone, as may be seen by referring to the map. The greater proportion by far is made up of umnmetamorphosed material. Nowhere can metamorphic rocks be traced from the contact with the serpentine for more than 25 feet without encountering unaltered material, but areas, isolated from other metamorphic rocks and from the serpentine by original material are common at distances much greater than this. The changes in these rocks are so profound and occur within such short distances that it seems hard to conceive of any action which might cause them. A chert, for example, may be unaffected here, but five feet away may be trans- formed into a massive glaucophane rock without ever a vestige of its original structure. The intensity of metamorphism has varied in various places, but observations show that it does not depend upon either the original material or its proximity to the intrusion within the aureole above described, The metamorphic rocks can be roughly classified on the basis of their original nature,although often this can not be definitely determined. All of the rocks have been somewhat similarly affected by metamorphism,and many of the same minerals ousnr in each. The chert offers the most direct gradation into its metamorphosed .m-"pg and will, therefore, be considered first, METAMORPHOSED CHERT. From the original cryptocrystalline, slightly colored, radiolarian chert described above, the first transformation is in a grammlation and recrystall- ization of the silica. In thin sections, the mosaic ground of a quartzite is visible. The radiolarian relic structure is lost and the rock, if colored, becomes pure white to colorless. No loss of definition between shale and chert laminae is visible, but minor cremulation and folding is seen in the field. This first stage may easily be attributed to normal contact action or even to a slight regional compression. Beyond this stage pneumatolysis is active, for there is a development of fine needles and sheaf-like tufts of microscopic blue and green amphiboles showing no definite orientation. (Plate X1). In the hand specimen, the rock is still white, With an increase in the amount of blue amphibole, the rock takes on a bluish tinge, and finally passes over into an amphibole schist composed wholly of interlocking fibrous blue prisms. The rocks so affected often occur in elongated outcrops, having a dyke-like appearance and bounded on either side by unmetamorphosed rocks. (Plate V1l). The blue amphibole in some rocks is glaucophane; in others, crossite, Both are recognizable under the microscope by their intense blue and violet pleochroism and small extinction angle, and each may be distinguished from the other by the orientation of the axial plane. They are of similar chemical composition (11) and the formation of one rather than the other in certain rocks cannot be explained, Actinolite has also been formed in some rocks and occurs,either as separate fibers, or else grown in erystallographic continuity with the blue afphibole, An illustrative occurrence of this kind is shown in Plate X11, in which the central naberial is actinolite and the bordering mineral is crossites Both show the emergence of Bxa= X . The inference here is that there has been a molecular interchange in the original actinolite, giving rise to the more sodic amphibole, crossite, by a variation in the composition of the solutions or gases carrying in the amphibole constituents. Somewhat similar occurrences have been described by Ransome (7) and Davis (8). The shale partings in the chert during such a transformation appear in the hand specimen at first to be less affected than the chert laminae, but where the rock has become a glaucophane (crossite) schist, its original character may only be inferred by the presence of the cremulated cherty bands not altogether replaced by the glaucophane (crossite). Another transformation of the chert has been to a dark bluish green to black garnetiferous amphibolite schist, The green amphibole appears to be actinolite or hornblende. The presence of a blue amphibole probably gives it its dark tone. Its original character is evident from narrow streaks of chert traversing the rock. A rock which appears to have once been a shaly member of the chert has gone over into a dark blue grey slaty mass. It is very fine grained and indeterminable in the hand specimen. Glaucophane or crossite probably gives it its bluish color, METAMRPHOSED SANDSTONE. The sandstone formation, similar to the period of recrystallization of the chert, passes through a preliminary stage of metamorphism in which a rough parallelism of grains is observed, appearing to be definitely due to the peridotite intrusive. The parallelism becomes more noticeable on approaching the serpentineyand near the southern end of the peninsula, it is definitely gneissic and so hard that it rings when struck with a hammer. Further away, as on the north-eastern slope of hill 780 (see map) no trace of parallelism can be detected. The products of further metamorphism of the sandy sediments may be described as the "acid metamorphics™ to distinguish them from the less acid types derived from the greenstone. The principal minerals formed are cuartz, muscovite and biotite, actinolite, glaucophane or crossite, epidote, garnet, rutile, pyriteyand veins of secondary albite, quartz,and calcite. The schist minerals are present in varying amounts in different rocks and by the increase of one or another over the rest, a multitude of schist varities may be produced. Such types as quartz-mica-schist, quartz- mica-garnet-schist, glaucophane- garnet-schist, glaucophane-actinolite~-schist and many others are present. The product of most extreme metamorphism appears to be a quartz-mica-schist with more or less garnet. The garnet is probably of the calcium-iron variety, andradite, » The acid schists are all ch:racterized by more or less cuartz and mica, and may roughly be separated from those noticeably lacking in these minerals, which have been termed basic and which appear to be derived from the greenstone. The conglomeratic phases of the sandstone have been similarly changed, and although in the outcrop the relic structure is at times still visible, in thin section the boundaries of the individual grains have been completely obscured by newly developed mineral aggregates. The shaly members become at first sericitic and then pass over into a’ fine grained tough green rock containing actinolite, gquartz,and mica. METAMORPHOSED GREENSTONE. The metamorphic products of the greenstone, which aan at times only difficultly be distinguished from the metamorphosed sandstones, ordinarily are less siliceous than the other schistsyand are to some extent characterized by the presence of chlorite and tremolite. The principal constituents of these schist members are actinolite, epidote, glaucophane, tremolite, chlorite, and garnet, present in varying amounts in various rocks and giving, therefore, an abundance of related types, all of which rather characteristically are without quartz and mica. Schistosity is less distinctly developed than in the sediments and often these rocks show a very lm outcrop. A typical suite of rocks showing the gradation from greenstone into a basic schist showed first the development of granularity in the rocks then a change to an actinolite schist with slight chlorite; then in addition to chlorite and actinolite, the development of biotite; next a tremolite-actinolite-chlorite schist; and lastly a schist containing akinoniie, glaucophane, epidote, pyrite, and rutile. (Plate X111). At other times the end product of metamorphism has been a pure tremolite rock and again a pure actinolite schist. Glaucophane apparently is less abundant in these rocks than in those derived from the arenaceous sediments, just as it is less abundant in the arenaceous sediments than in the cherts. On the south point of Tiburon, crossite occurs associated with greenstone; not, however, as a constituent of the rock, but as secondary coatings obviously deposited by solutions or gases upon the fracture and cleavage surfaces of otherwise fresh rock unaffected by metamorphism. Strangely enough, no rocks containing lawsonite were recognized in this portion of the peninsula, although it is typical of the region and found further north. CONCLUSION. From the foregoing it seems very difficult to ascribe the production of the metamorphic rocks of Tiburon to any other cause than contact action with the serpentine. There is a possibility that certain schistose structures in the sandstone and the first recrystallization of the chert, above described, may be due to regional forces: but even this minor part of the whole, from its restriction to the borders of the serpentine dyke: must be attributed to the action of that body, either during intrusion or during serpentinization. The production of a highly sodic amphibole in the sandstone and the greenstone may, according to Smith (9), be a result of simple recrystallization of elements already present,by normal contact action. Granted that there is sufficient soda in the feldspars of the arkose and the greenstone, still the conversion of a chert which is at least 95% silica into a pure glaucophane (crossite) rock,containing tut 50% silica,and nearly 10% soda as well as other bases, may not be explained in this manner. Pneumatolysis seems to be absolutely necessary for the production of such silat, Evidence in addition to the above lies in the observations of coatings of blue amphibole upon the fractures and cleavages of the greenstone, above described; in the linear elongation in a dyke-like fashion of certain glauco- phane (crossite) schists derived from the cherts, further indicating passage ways for solutions or gases, (Jee Plate V1l); in the extremely sporadic and independent distribution of the various schists,already referred to and describedgy and in the production of certain characteristic minerals in all the metamorphic rocks regardless of original composition. Regarding the source of the soda in these rocks, three possibilities present themselves. As suggested by Smith (9) it may have been present in the original rocks. In order to account for its presence in the metamorphosed chert on this assumption, it must be further supposed that solutions leached certain necessary constituents from the sandstones and greenstones, and deposited them in the chert member, forming the blue amphiboles. A second possibility is that the soda came from the peridotite mass. Since the metamorphic rocks are obviously associated with the serpentine, such a conclusion would seem most probable were it not for the fact that no soda bearing minerals are found in the serpentine. This, however, does not preclude the possibility of there being such minerals present. Thirdly, there is the possibility of heated vapors coming, not from the peridotite mass, but from the more acid magma of which this is a differentiate, and carrying with them, through the fissures and cracks made during the injection of the peridotite, mineralizers high in soda, SUMMARY The Tiburon Peninsula presents an elongated tongue projecting into San Francisco Bay whose rocks are now in the form of a syncline, the axis of which is approximately parallel to the elongation of the peninsula and 1s occupied by a peridotite dyke now serpentinized. The rocks on the peninsula are of lesozoic age and have been correlated with the Franciscan series. They comprise, in the order of their formation: (1) An arkose with some conglomeratic and shaly facies. (2) A radiolarian chert, thinly interlaminated with siliceous shale. (3) A greenstone in the form of flows, dykes and sills. (4) 4 peridotite dyke now completely serpentinized. The peridotite dyke has apparently been the cause for the metamorphism of all the older rocks,which occur about its periphery. The type of metamorphism here represented is exomorphic pneumatolytic,with the production typically of high soda bearing minerals, especially glaucophane, in rocks regardless of their nature, The ultimate origin of the soda can not be definitely stated. A classification of the metamorphic rocks on the basis of their original composition shows that the chert§ after recrystallizatiom of the silica, pass into pure glaucophane or glaucophane-actinolite-garnet rocks; the sandstones give a greater variety of schists which are all, however, characterized by quartz and mica together with other constituents; and the greenstones become metamorphic rocks lacking in quartz and mica but containing generally chlorite and tremolite, together with other minerals in various proportions. BIBLIOGRAPHY (1) Lawson, A. C., San Francisco Folio, U. S. G. 5S. Folio #193. (2) Merriam, J. C. 20th Ann. Rep. U. 5S. G. Se Part 2, pp. 338-339 1896. (3) Davis, C. H, Jour. Geol. Vol. 21, pg. 453. 1913. (4) Louderback, G. Jour. Geol. Vol. 13, pg. 550 1905. (5) Davis, BE. F. Dept. Geol. Bull. Vol. 10 No. 1, 1918. (6) Harder, E. C. G. S. Bull. 427, pp. 159-167. 1910, (?) Ransome, F. L. Dept. Geol. Bull. Vol. 1, pg. 193. 1893. (8) Davis, EB. F, Dept. Geol. Bull. Vol. 11, pg. 235. 1918. (9) Smith, J. P. Am, Phil. Soc. Vol. 45, pg. 183. 1906. (10) Capps, S. R. Geol. Vol. 23, pg. 45. 1915. (11) Murgoci, A. Dept. Geol. Bull. Vol. 4, pp. 359-396. 1906. HOU oe 2) songdoiictyyy 4 saoysuzals £424, D YoYs dlragspuog A ANS AT TL vr BRAT >, re ~ Dn EN SL 8 a ol A 2 J7737 oo5 LEGEND Greenstone Serpentine 2 — a NN SF WN 2 2 )) 3) / Survey Mago. U.S. Coost & Geodetic SCALE : Fin = 4 rte. The THE TIBURON PENINSULA. from [IH ai rs AS papal BITAT I a bon we Rek . ¢ mp ads —r TW ren. ~~ PLATE 1. : | PLATE 11. CHERT OUTCROP. Showing regular narrow SANDSTONE-SERPENTINE CONTACT. Showing interlamination of chert and shale, and minor depression formed and rugced outcrop characteristic crenulation of beds. of serpentine. (at right]. PLATE 111. Fige 1 SERPENTINE OUYCROr. Massive or boulder-like facies. : RELATIONS AT STA. 8S. Topography reflects underground structure but beds stand vertical at serpentine contact. PENTINE OULCROP. Foliated or slickensided facies. RELATIONS AT STA. 8. Near view of upturned chert beds. GREENSTONE. Showing pillow structure. GREENSTONE. Hammer rests upon a chert inclusion. PLATE V1ll. GLAUCOPHANE SCHIST. Showing dyke-like elongation of outcrop. RADIOLARIAN CHERT. Showing remains of Radiolaria. Traversed by a vein of quartz. GREENSTONE. X-Nicols. Laths of basic : SERPENTINE. X-Nicols. Shows bastite labradorite in a ground mass of andesine and augite. structure. Enclosing pseudophenocrysts is mesh structured serpentine after olivene. olivene. 2 anclosing structure. rite aug a 2 = O z a = Oo wid or a = Co ad LZ < p= = 4 ic o~ wo ths of bas La labradorite PLATE Xl. PLATE X11. METAMORPHOSED CHERT. First development METAMCRPHOSED CHIRT. of glaucophane in fine tufted crystals. actinolite with crossite developing in cleavages and on border. Lower crystal is crossite. Upper crystal is RETAKE OF PRECEDING FRAME yo - ATE a dd developmen glaucophane in Lye YNTPT TY Ahi HHS ) MIA AALL 4d dl