234 The American Geologist. April, 1894 
morphism and the other c6inparatively little, and the marked differ- 
ence in dip of the two bodies, taken as a whole, strongly suggesl an 
unconformity . 
Mariposa Formation. 
The Mariposa slates constitute the most definite of the for- 
mations that have been recognized in the auriferous slate 
series. Where studied by the writer to the south of the 
American river this formation consists almost wholly of black, 
clay slates, with layers of diabase-tuff and a little conglomer- 
ate, made of siliceous pebbles from the rocks of the Calaveras 
formation. The Mariposa slates were considered of Jurassic 
age by Gabb and Meek, who determined the following 
fossils : 
Ammonites* colfaxi. Pholadomya ? orbiculata. 
Aucella erringtoni. Belemnites paciflcus. 
Aucella erringtoni, var. linguiformis. 
The l". S. G-eol. Survey has collected fossils at a number of 
localities in the Mariposa slates. The most prolific of these 
is the Texas ranch in Calaveras county. The following fos- 
sils were obtained here by Mr. ( '. I). Voy and Dr. Cooper 
Curtice, and determined by Dr. White and Prof. Hyatt. 
Aucella erringtoni. Amaltheus. 
Perisphinctes. ( !ardioceras. 
Belemnites pacificus. 
There is some difference <>r opinion as to the age of the Mariposa 
slates. Prof. Hyattf is of the opinion thai the Aucella' and. the Ammo- 
nitime id' the genera Cardioceras and Perisphinctes, from Mariposa. 
Calaveras, and Tuolumne counties prove the Upper Jurassic age of the 
slates. The species of Cardioceras and Perisphinctes have the common 
characterisl ics of similar forms found in the Upper Jura of Russia, and 
these occur in association with similar forms of Aucella 1 . The species 
of Aucella' differ, as a whole, from those of the Knoxville slates in being 
almost invariably ornamented with radiating stria?. Only one species, 
Aucella erringtoni, var. arcuata approximates in outline to the well- 
known narrow form of Aucella piochi. The conclusion is reached that 
the fauna of the Mariposa beds is Upper .Jurassic and older:): than 
the Knoxville formation, which is usually regarded as Neocomian. 
Dr. White considers the Aucella 1 . in the Mariposa beds and those in 
the Knoxville beds of t he ( 'oast ranges to belong to t he same species, al- 
though a ven variable one. He therefore concludes that both series of 
-Prof. Hyatt states that this ammonite belongs to the genus Perisphinc- 
tes. The specimen is now in Cambridge. 
! Am. .lour. Sci.. vol. 17. p. 1 13. 
{'The abstract in the Am. .lour. Sci.. says younger, hut is a mistake. 
