246 The American Geologist. April, 1894 
Carboniferous, while the Mariposa slates are <>r late Jurassic, <>r accord- 
ing to Dr. White, pf early Cretaceous age; and, as tin- Mariposa slates 
are directly in contacl with the Carboniferous rocks, there is plainly a 
long time hiatus. 
The narrow streak of the Mariposa slates referred to in the foregoing 
paragraph seems certainly to have been deposited in a trough between 
pre-existing ridges of the Calaveras format ion and to have been folded 
up with the rocks of that formation at the time of the post-Mariposa 
upheaval. The slates of both the Calaveras formation and of this nar- 
row bell of Mariposa slates dip at a high angle to the east, but as before 
noted there is a slight diversity in strike. 
Evidence similar to the above may be obtained by noting the strike 
of t he two main bells of the Mariposa formation, which were first de- 
scribed in I his journal, vol. 11. p. 308. The western belt of slate main- 
tains a general northwesterly strike from near Folsom on the American 
river to Merced falls on the Merced river; the eastern belt, on the other 
hand, maintains a northwesterly strike from its south end near Mariposa 
to near Plymouth in Amador county, where it is flexed, pursuing thence 
a north course to Placerville, again bending northwesterly to the north 
of that town. The result is that the two belts, which are nearly paral- 
lel as far north as the Cosumnes river, diverge at an angle of about 30° 
to the north of t hat river. 
The geographical distribution of the genus AueeUa also indicates that 
a land area existed in the region now occupied by the Sierra, Nevada, at 
the time the Mariposa beds were being deposited, for the genus is not 
found in the .Jurassic deposits of the higher part of the range at Sailor 
canyon, nor in the highly fossiliferous Jurassic beds of Mt. Jura, in Plu- 
mas county. 
The Hinchman tuff, which is the highesl horizon recognized by 
Diller and Hyatt, is considered to represent not the very top of the Ju- 
rassic, but the Corallian of Europe, while the Mariposa beds are re- 
garded, I think, by Hyatt as representing the uppermost Jura, so that 
our present knowledge suggests that all of California and Nevada to the 
east of thi' area of the Mariposa beds has been above water since the 
close of the Hinchman tutf (Corallian) epoch. 
This distribution of the genus appears to favor the hypothesis of Dr. 
White and Mr. Becker that the Mariposa and Knoxville beds are of the 
same age. If it be supposed that a continuous series of Aucella-bearing 
beds were deposited on the western border of this pre-Mariposa land 
area, and that I hese deposits extended over the area now occupied by 
the Sacramento- San Joaquin valley and into the present Coast 
ranges, and that the orographic movements that uplifted, folded and 
metamorphosed the Mariposa beds were not strongly felt to the west of 
the ureal valley-, it could be held that the Mariposa and Knoxville beds 
are merely different port ions id' this continuous series, the slaty struc- 
ture and metamorphism of the Mariposa beds being due to their being 
in the zone of the intense dynamic movements that formed the Sierra 
Nevada, while the Knoxville be<is. being ou1 of this zone, were less dis- 
