196 QUICKSILVER DEPOSITS OF THE PACIFIC SLOPE. 



proper. The fossiliferous Mariposa beds already mentioned occur near the 

 southerly end of this narrow portion of the auriferous area. 



Previous to the discovery of fossils on tlie Mariposa estate in the series 

 which I sliall call the Mariposa beds, Professor Whitney and his associates 

 had collected in the Coast Rang-es Beleinnitcs, a .shell determined as Iitoce- 

 ramus I'iochii, and some others, from the strata which I liave entitled the 

 Knoxville beds. ]\[r. Gabb described them as Cretaceous forms.' Some 

 years after Mr. Meek liad referred the Mariposa beds to the Jurassic Mr. 

 Gabb redesciibed Inoceramus Piochil as AuceUa Piochii,- a chang-e of genus 

 which I understand to be unquestionably correct. This correction appeared 

 to me at the very beginning of this investigation of great importance to 

 the stratigraphy of the State, for through it the fauna of a large part oi" the 

 known rocks sup[)osed to belong to the Shasta group of the Cretaceous ac- 

 quired the strongest resemblance to the fauna of the Mariposa County Ju- 

 rassic. Indeed there seemed scarcely room left for a distinction; i( AuceUa 

 is distinctively Jurassic, the AiiceUa-henv'mg beds of the Coast Ranges must 

 be members of that system, while if these AuceUa beds are Cretaceous ^m- 

 ceUa is not a distinctively Jurassic genus, even in the State of California, 

 and i\Ir. ^Meek's principal reason for assigning' the Mariposa beds to the Ju- 

 i-assic is .shorn of its validit}'. Dr. White afterwards fully confirmed this 

 view, and after examination of Meek's types, together with new and better 

 .specimens which we collected, he is unable to draw any specific distinc- 

 tion lietween the AuccUa of tlie ]\Iariposa beds and that of the Knoxville 

 bed.s. 



Professor Whitney states that, while the Mesozoic age of the Mariposa 

 beds is proved by their fossils, the Pre-Cretaceous age of these strata is dem- 

 onstrated by their stratigraphical relations. Professor Whitney has indeed 

 shown that Cretaceoas strata rest unconformably^ upon the upturned edges 

 of the auriferous slates along the foot-hills of the Sierra at several points ; 



'Geol. Survey California, Pala'outology, vol. I. 



-Ibid., vol.2. 



^lam perfectly satisfied of the existeuce of this uon-couformity, tliough the loealities where the 

 Cliicobeds Ii.ive been found resting on the uptnrued edges of the auriferous slates are not near those 

 in which Mesozoic fossils have been found in the older rocks. The Chico beds, where they occur along 

 the foot-hills, have suli'ered little if at all from the Post-Miocene uplift in the Coast Ranges aud are 

 nearly horizontal. The Mariposa beds are almost vertical. 



