SUCCESSION OF MESOZOIC LAVAS. 439 



The limestones of the series are not continuous and are frequently 

 absent, and they occur in places elsewhere than in the third member of 

 the series; hut they arc characteristically frequent and extensive in this 

 member. The serpentines are also not constant in the second member, 

 or the diabase or greenstone in the lowest member; but there is no very 

 large area of exposure of the former without serpentine or of the latter 

 without greenstone or diabase. Serpentine sometimes occurs in small 

 proportions in the lower subgroup, and south of Sutter creek the green- 

 stones are not entirely confined to the lower subgroup, hut occur in 

 small proportions among the serpentines and slates accompanying them 

 of the upper subgroup, and possibly among the thin slates at the head of 

 the series. There are also in the more southerly exposures of the thin 

 slates some sandstones, and at one place near Montezuma, between the 

 Stanislaus and Tuolumne rivers, I have seen among them a fine con- 

 glomerate. I have not found limestone among these thinly laminated 

 slates except in the district described-, between the East branch of the 

 North fork and the Middle fork of Feather river. The non-chloritic 

 character of the diabase in a part of the exposures shows a difference in 

 degree or kind of alteration, and there are other minor differences. Still, 

 there are enough distinguishing characteristics of the several subdivisions 

 of Mesozoic rocks common to each throughout the areas of exposure to 

 render it readily identified. 



The division of the Mesozoic rocks into upper and lower subgroups 

 simply brings out to view the characteristic eruptive activity and depo- 

 sition at the different horizons. The principal eruptives in the pre- 

 Meso/.oic series are granites; in the lower Mesozoic, diabase or green- 

 stone, products of alteration of a medium basic lava: in the upper 

 Mesozoic, serpentine, ;i product of alteration of basic lavas. The succes- 

 sion of lavas in the Sierra in Mesozoic time is similar in one respect to 

 thai of Tertiary time, when the principal outflow of basalt followed the 

 principal outflow of less basic lavas. 



I have not attempted to give the thickness of the Mesozoic series or 

 any of its members.;!.- it Is obscured by faulting: but data are accumu- 

 lating which will, I trust, make it practicable to eliminate the errors 

 from this source. The whole series is certainly several miles thick. 



Fossil Horizons. — In three of the four natural divisions of the Mesozoic 

 series fossils haye be. n found, namely, in the thinly laminated shales at 

 the head of the series {Aucella, Belemnites, etc, on the Merced, Mariposa 

 county); in trie slates ami lime-tones with greenstones (crinoids with 

 pentagonal stems, etc. at the northern end of the range): and in the 

 lowest division, consisting of slates and diabase or greenstone i Ammonites 

 colfaxii, on the ( 'eiitral Pacific railroad i. 



