3904 THE JOURNAL OF GEOLOGY. 
east base of the Bear Mountains, and occurs in an augitic slate. 
In the collection of Stanford University there are two specimens 
of ammonites from the same horizon as the Cherokee Creek 
specimen, although the localities are further north on the area of 
the Placerville sheet. The rock in which the fossils occur is a 
distinct augite-porphyrite tuff, and would ordinarily be called 
greenstone. The writer is indebted to Professor J. P. Smith for 
the following information concerning them: One ammonite is 
Perisphinctes conf. colfaxi Gabb sp., and was found by Mrs. M. J. 
Gates at Huse’s bridge over the Cosumnes river; the other is 
Perisphinctes sp. or Olcostephanus, and was found in a bowlder near 
Nashville, Eldorado county, by J. C. Heald. Both species indi- 
cate the Upper Jura. 
According to Diller,? the Hinchman tuff at Mt. Jura in Plumas 
county contains lapilli, affording evidence in that section of vol- 
canoes during the time of deposition of the tuff (Upper Jura). 
Mt. Jura is perhaps one of the best places in the range for the 
chronological study of ancient volcanic materials, both on 
account of the greater freshness of the rocks and the abundance 
of fossils. 
As will be noted on the geologic map, the large area of por- 
phyrite and amphibolite widens going north. On the Smartsville 
geologic map, now being published, these rocks may be seen to 
occupy a large portion of the surface of the country. They have 
been studied mostly by Mr. W. Lindgren, who agrees with the 
writer in regarding them as chiefly surface eruptions and proba- 
bly Jura-Trias in age. These rocks, largely augite-porphyrites 
and their tuffs, are presumed to have covered, as with a mantle, 
the underlying Paleozoic formations. There are some streaks 
of slates among the eruptive masses, but these have not in the 
Smartsville area afforded any fossils. However, during the past 
season, in the north extension of the same area, ina belt of clay- 
slate interbedded with augite-breccia and tuff, fossil plants were 
collected by T. W. Stanton. The exact locality is by the stage 
™ Fourteenth Annual Report U.S. Geological Survey, p. 453. 
2 Bull. Geol. Soc. Am.,Vol. III., p. 373. 
