272 QUICKSILVER DEPOSITS OF THE TACIFIG SLOPE. 



and a little impure limestone. The beds stand at many angles, but their 

 dip is .usually very high, while the prevalent strike is in the direction of the 

 ranges. Fossils are abundant at a few points, but are not very generally 

 disseminated. By far the most frequent and the most important forms are 

 two species or varieties of Aucella. These were not distinguished by Mr. 

 Gabb, Avho collected specimens here and gave them the name A. Piochii, 

 but Dr. White considers the more robust form as .1. concentrka and the 

 more slender as A. mosqucnsis. These and the accompan^nng fossils have 

 been fully discussed in Chapter ^', and the conclusion was there reached 

 that the beds carrying them are close to the line of division between the 

 Jurassic and Cretaceous formations, but are probably to be considered as 

 the earliest Cretaceous, and therefore as belonging to the Neocomian period. 

 The studv of these fossils when first collected led me to the belief that the 

 beds carrying them could not be separated from the slates of the gold belt, 

 which also carrv Aucella. This conclusion was afterwards full}' confirmed 

 l)y Dr. Wliito. 



Metamorphk rocks. — Tlic Coast Kaugcs arc so scantily supplied with fossils 

 that tlie determination of these beds and their correlation with those of the 

 Sierra Nevada are matters of much interest; but of no less interest is the 

 fact that this district affords abundant opportunities of tracing the passage 

 of these beds into the metamorphic rocks. The microscopical evidence of 

 these transitions has been set forth at great length in an earlier portion of 

 this memoir, but the structural relations have been only briefly referred to. 

 These are of great importance for two distinct reasons. One of them is 

 that eminent geologists deny that large areas of ordinary sediments are 

 converted into crystalline rocks and serpentine by secondary processes ; in 

 other words, they deny the theory of regional metamor2)hism. The second 

 reason for a minute description of the occurrence is that the results of mere 

 microscopic examinations of collections are not altogether trustwoi'thy. 

 The phenomena which specimens and slides from complex areas present 

 are so multifarious that it is nearly always possible to draw various plausi- 

 ble conclusions from them. Specimens may often be so arranged as to sup- 

 port arguments either for connecting the most diverse rocks by transitions 

 or for separating varieties which are in reality' closely allied Wlien due 



