78 The American Geologist. Fei)riiary, 1393 
many changes liave been made. .Jules Marcou has lately as- 
serted that the older genus Avicula has been confused with the 
Aucella. and that the rocks in which it is found are not Creta- 
ceous but much older.* 1 suppose he refers to the attempt to 
include in the Cretaceous the supposed Aucella l>earing strata 
of the Mariposa beds which were classed b}^ (labb in the Jur- 
assic. Granting that the classification is perfectly correct, Dr. 
White acknowledges that the Aucella is not unequivocal in its 
signification and says: "I do not think that any satisfactory 
evidence has yet been presented to show that the genus Aucella 
is exclusively confined to either the Jurassic or the Neocomian, 
and I know of no reason why we may not expect to find species 
of it in both Jurassic and Neocomian strata. "t Again speak- 
ing of the Mariposa beds on the Merced river, says: -'The 
strata have an almost vertical dip, and they are plainh' an in- 
tegral part of the great Auriferous Slate series." — "The 
great x\uriferous Slate series is an immensel}- thick one, and in 
northern California it is known to include strata of Carbonifer- 
ous age. '■ + 
There can be no question but what the genus Aucella is one 
of the most characteristic lower Cretaceous fossils in California, 
and it is fully proved that these unaltered beds rest unconform- 
able upon the Metamorphic or Auriferous series. Several ex- 
amples of this were cited in my former paper. It has also been 
fully proved by Mr. Diller and others that there is no important 
nonconformity between the lower and upper California Cretace- 
ous. Consequentlv the nonconformity shown where the Chico 
rests directl}' on the Auriferous series in the Sierra Nevada, may 
with certainty be referred to a pre-Cretaceous upheaval. 
The interesting discoA'eries of J. S. Diller in the Taylorville 
region have an important bearing on the question under discus- 
sion. He found there the most complete series of Jurassic beds 
yet known in the United States, lower, middle and upper Jura 
being represented. With regard to the separation between the 
upper Jura and the Cretaceous Mr. Diller says: '-So far as yet 
known, on the fortieth parallel the rocks next 3ounger than the 
Taylorville Jurassic are the Knoxville beds of the earlier Cre- 
*Geological Maj) of tlie United States. 
tBull. U. S. Geological Survey, No. 15, page 26. 
tBuU IJ. S. (Teological Survey, No. 1, p. 25. 
