Geology of the Mother Lode gold belt. — Fairbanks. 211 
buckeye and poison oak are the most prominent of the smaller 
growths. 
The ascent from the plains of the San Joaquin valley is gradual, 
each succeeding valley being a little higher. South of Mariposa 
county the slope up to the crest of the Sierras is much more rapid, 
and with but little intervening sedimentary strata however the sed- 
imentary strata widen quite abruptly in Mariposa county, and in 
El Dorado county the} r reach a width of forty or fifty miles. 
The Mother Lode occupies about the centre of what is called by 
Whitney "the auriferous slate belt." The nearly level Tertiary 
rocks rest on the edge of the upturned slates which are penetrated 
by many dikes and granite masses. Farther east the slate finally 
disappears and granite becomes the prevailing rock. However, it 
must be borne in mind that over a part of the western slope, what 
is usually called the metamorphic area is formed largely of truly 
eruptive rocks, which have become so much obscured in character, 
through various metamorphic agencies, as to appear bedded and 
of sedimentary origin. 
The age of none of these rocks, except the horizontal strata, 
has been positively decided ; a part of these are late Cretaceous 
and a part are Tertiary. It is probable that the upturned slates 
are of Jurassic age or early Cretaceous as affirmed by G. F. 
Becker, while the granite by which they are upturned, intruded 
and metamorphosed, can hardly be considered Archaean though it 
has often been mapped and described as such. 
As regards the stratigraphical relations of the different members 
of the series as well as the paleontological evidences more will be 
said at the close of this article. 
So scanty are the fossil remains over a large part of the foot- 
hills, and so violent have been the disturbances of the strata that 
the region is one of uncommon difficulty. However within the 
last year new locations of fossils have been discovered in the lime, 
stone areas along the lode where hitherto all efforts in that 
direction have been futile. Whatever may be the character and 
value of the gold deposits in Fresno county it is certain that the 
southern end of the Mother Lode lies in Mariposa county. A 
great mass of eruptive granite extends down from the high Sierras, 
cuts across all the other formations, both sedimentary rocks and 
dikes, and terminates five miles west of the town of Mariposa. 
The dikes as a usual thing do not extend up to the granite, but 
