JURASSIC. 555 



is probably Cretaceous. (See table of the geologic formations of the Coast Range 

 of California in the vicinity of the bay of San Francisco, p. 816, Chapter XVII.) 



For a description of the Franciscan, we may refer to Lawson's original account.^^* 

 According to that statement, it consists of the following petrographic elements:" 



(1) A basal formation of conglomerates, coarse grits, sandstones, shaly sandstones, shales, 

 and argillaceous limestones, exposed in the vicinity of San Pedro Point. These rocks require 

 no detailed discussion. 



(2) The San Francisco sandstone, the dominant sedimentary formation of the series, con- 

 sisting of a moderately fine grained sandstone, fairly uniform in character over large areas, with 

 subordinate beds of shale and conglomerate. The sandstone is uniform, not only in its lateral 

 extension, but vertically for great thicknesses. In the older writings it has commonly been 

 referred to as the San Francisco sandstone, and the term is here adopted from that usage. It is 

 interbedded with the formations 3-5 which are named below. 



(3) Foraminiferal limestones. 



(4) Eadiolarian cherts. 



(5) Volcanic rocks, including basaltic lavas, diabases, pyroclastic accumulations, etc. 

 Besides these there are intrusive rocks of a corresponding character, some of which are probably 

 connected with these extravasations, and also intrusive peridotites and pyroxenites, now ser- 

 pentinized. 



In addition, there are certain metamorphic schists, which are the products of the local 

 alteration of the sedimentary or volcanic formations of the series and, according to the writer's 

 interpretation of them, do not constitute a separate formation. There are also a few patches 

 of a pecuhar veinhke rock associated with the San Francisco sandstone, but of rather uncertain 

 genetic relations. In his field notes the writer has been in the habit of referring to this as 

 silica-carbonate sinter, and it will be further described under that designation. 



J 10. SIEBBA NEVADA, CALITOBNIA. 



Along the western base of the Sierra Nevada in California the Mariposa slate 

 forms three narrow bands. The strata are prevailingly clay slates, which are 

 sandy and contain pebbles of rocks from the Calaveras formation (Carboniferous). 

 Tuffs from contemporaneous porphyrite eruptions also occur in them. The fossils 

 of these beds, such as Aucella and Perisphinctes, indicate a late Jurassic age. 



In the folio describing the Sonora quadrangle Turner and Ransome *^^ give a 

 detailed account of the characteristics of the rocks in the three bands in which the 

 formation outcrops, especially as observed at localities within that quadrangle. 

 Lindgren ®^^ describes a more northern occurrence of the Mariposa in the Colfax 

 quadrangle, where it succeeds the Sailor Canyon formation (Triassic). He states 

 that it consists of black shales or slates, usually not very fissile, alternating with 

 dark-gray sandstones of coarser or finer grain, and a great number of conglom- 

 erate beds. The rocks are tuffaceous and contain much iron. The conglomerate 

 beds, though rarely very thick, are very abundant and contain pebbles of chert, 

 quartz, slate, and limestone evidently derived from the older Calaveras formation. 

 The Mariposa was clearly deposited in a gulf or shallow bay, the conglomerates 

 indicating immediate proximity of the shore line. 



The fauna contained in the Mariposa formation has been discussed by Hyatt ^^^* 

 and by James Perrin Smith. ^'^ 



« For a detailed description of the subdivisiona see the work cited. 



