Geology of the Mother Lode gold belt. — Fairbanks. 219 
different locations ; the same character of ore is rich in one spot 
and poor in another, without any apparent reason for it. 
Any one mining district is apt to be characterized by certain 
peculiarities, and a study of these is the best guide to go by in 
that district but they may be misleading in another. 
Very little that is new can be said concerning the age of the 
metamorphic rocks inclosing the lode. The range of opinion has 
been very great. T. S. Hunt would carry them back as far as the 
Huronian, while Gr. F. Becker believes them to be of the same age 
as the Knoxville group of the coast range, that is Neocomian or 
lower Cretaceous. The rocks have been generally classed as Ju- 
rassic on account of the presence of one or more species of the 
genus Aucella. This genus has a narrow range, not being known 
earlier than the Jurassic. It has been found in abundance in 
strata of the uoast range, proven to be Neocomian, hence, doubt is 
thrown on the generally accepted belief. Some other fossils have 
been found in the slates but none so characteristic as the Aucella. 
Triassic as well as Carboniferous fossils have been found in the 
northern Sierras, but no one has yet been able to correlate these 
formations, either limestone or slate, with similar formations in 
the middle Sierras. The evidence of fossils recently found in 
limestone in Tuolumne and Calaveras counties is supposed to favor 
the Carboniferous rather than the Jurassic. The fossils are few 
and quite fragmental, and it seems to me that the evidence is not 
yet sufficient to classify the limestones of the middle Sierras as Car- 
boniferous. As far as all observations have } r et been carried there 
has been found no unconformity in the sedimentaiy rocks of this 
region ; they appear to have been tilted up en masse. 
Whitney says that a carefully constructed section along the line 
of the Central Pacific Railroad furnished no proofs of folds. Since 
limestone occurs on both sides of the Mother Lode how can it be 
possible that it should be Carboniferous while the black slates of 
the lode are Jurassic or Neocomian ? If we suppose a downward 
fold the slates might occupy the center and the strata on either 
hand be similar, the older, farther awaj\ 
The dip of the rocks is nearly always at a high angle toward the 
mountain range. From every appearance I believe this represents 
not simply a tilting up but an overthrow. This being the case, 
the younger strata should be found in the lower foothills, the 
older, higher up. 
