FOSSILIEEUOUS LIMESTONES OF THE SIERRA. 425 



MESOZOIC Hocks. 

 PRINCIPAL HI Vis joys. 



The Mesozoic group includes both sedimentary and eruptive rocks. 

 The sedimentary rocks consist principally of slates often altered to 

 quartzites, with, however, some limestones. The eruptive rocks may 

 naturally, though rather roughly, he distinguished as medium basic 

 lavas altered to diabase or greenstone, and very basic lavas more or less 

 completely altered to serpentines. Both kinds are still further frequently 

 altered to quartzites. 



The whole group naturally tails into two subgroups, a lower and an 

 upper one. The lower subgroup is characterized by a large proportion 

 of the eruptive greenstones or diabases, while the upper one is character- 

 ized by deposits of serpentines, which in places attain enormous thick- 

 ness. The proportion of eruptive matter in both subgroups varies 

 exceedingly, and there is occasionally found a little serpentine in the 

 lower division and greenstone in the upper one; but as a whole the two 

 subgroups are characterized as stated. 



Right at the confines of the two subgroups, but falling most naturally 

 into the lower one. is a series of limy slates and limestones. These 

 limestones are fossiliferous. The most numerous remains arc of crinoi- 

 dal stems, and, as hereafter shown, some of them belong to Pentacrinus 

 or an allied genus, and cannot be of earlier age than Jurassic. We 

 have, therefore, as a, lower limit for the lower subgroup of Mesozoic 

 rocks of the Sierra, the base of the Jurassic. They may. however. 

 belong higher in the series. At the top of the upper subgroup is a long 

 series of thinly laminated slate-. I have found no fossils in these slates 

 within the district of my more detailed examination represented on the 

 accompanying -ketch map lying between the North and Middle forks 

 of the Feather; but comparison with exposures of similar slates south 

 of Merced river(iu Mariposa county) and at intermediate points proves 

 conclusively thai they are of the same horizon as the A ucella-b earing 

 slates which Whitney, on the identification of F. B. Meek, determined to 

 he Jurassic,* and which White places on the confines of the Jurassic 

 and Cretaceousf and Becker assigns to a higher horizon in the Cre- 

 taceous ' post-< rault). 1 



The fossils al these two horizons, one in each Mesozoic subgroup, show 

 that the whole group is above the base of the Jurassic, and this is con- 

 firmed by an ammonite which, as hereafter shown, occurs al still another 



*Geologj ■'!' California, vol. i. (8G5, p. J.'''.. 



>i. Survey, no. I :.. I 85, p. 26. 

 I. Geol. Soc. Am.. \ ol. 2, 1890, pp. 201 



