254 J. p. SMITH AGE OF THE AURIFEROUS SLATES. 



C. alternans described by P. de Loriol * Thus the Russian and the Cali- 

 fornian specimens may represent extremes of the same species. If this 

 Californian species should not be identical with C. alternans, I would 

 suggest the name Cardloceras whitneyi for it. 



Cardioceras alternans is confined to the upper Oxford in Germany, but 

 in Switzerland and in Russia ranges up into lower Kimmeridge — that is, 

 into the lower part of the zone of Aucella hronni. Thus the beds con- 

 taining this fossil must be of Upper Jurassic age and between the upper 

 Oxford and the middle Kimmeridge. The accompanying Aucellx being 

 closely related to both A. hronni and A. pallasi makes it probable that 

 they belong to the higher part of the series, that is, to the lower Kim- 

 meridge. 



Perisphinctes (?). 



One fragment in the slates is entirely different from the common Car- 

 dioceras, and seems to have the characteristics of Perisphinctes, but it is 

 very doubtful. 



The fauna of the slates of Texas ranch, Calaveras county, is plainly 

 the same as that of the Mariposa slates, and any evidence of the age of 

 the one applies to the other also. 



Mariposa Slates. 



In the first part of this paper the opinions of various writers have been 

 given as to the age of the Mariposa slates, which, upon supposed pale- 

 ontologic evidence, have been assigned to various periods, ranging from 

 the Trias to the Gault ; but from what has been said of the slates at 

 Texas ranch it is seen that the Aucella beds of Mariposa are of the age 

 originally assigned to them by Gabb,t although Meek recognized their 

 position more accurately by comparing the AucellaX to a European species 

 of known position. 



There can no longer be any doubt that the Mari|)osa beds belong to 

 the lower part of the Upper Jura, and are thus probably the youngest 

 Jurassic deposits known in North America, with the possible exception 

 of the Corallian described from mount Jura, Plumas county, by A. 

 Hyatt. § 



Professor Hyatt || shows that the Jura of the Black hills and adjoining- 

 regions belongs to the Callovian or lower Oxford, but that in that time 



*Abhandl. Sehweizer Palaont. Gesell., vol. iii, " Monographie Zone A. tenvilobatiis de Baden," 

 p. 20, pi. i, figs. 17 and 18. 

 fProc. California Acad. Sci., vol. iii, p. 172. 



I Geol. of California, vol. i, p. 479. 



g Bull. Geol. Soc. Am., vol. 3, p, 408. 



II Op. cit., p. 409. . » 



