330 - JOURNAL OF THE PLYMOUTH INSTITUTION. 
8rd. Deposits in which the gold of ancient river systems have 
been re-distributed by modern streams. 
Veins containing a considerable amount of metallic sulphides, 
particularly iron pyrites, the most auriferous. 
The auriferous gravels of 2nd class, vary in thickness to 120 
feet, a common depth; and when, as sometimes the case, protected 
by a capping of basalt, their thickness not unfrequently exceeds 
250 feet. One half of the gold of the country being produced 
from these deposits. 
The auriferous quartz, the principal source of gold, is enclosed 
within the slates and other metamorphic rocks. These masses of 
slate form a belt having a length of about 300 miles, and an 
average width of 50 or 55 miles, and have sandstone on either 
side of them to the east and west. 
Passing off the sandstone on to the granite of the Sierra Nevada, 
in which is situated the Yosemite Valley, a description of some of 
the distinguishing features of this locality was given; the approach 
to verticality of its walls; next, their great height (3,300 feet), 
not only absolutely, but as compared with the width of the valley 
(half a mile); and, finally, the small amount of débris or talus at 
the bottom of the walls. 
The characteristics of the granite masses between the Yosemite 
Valley and the Mono Basin, were described as showing remarkable 
effects of glacial action in times long past. The living glacier of 
Mount Lyell, 13,700 feet above the sea level, was mentioned as 
being on the northern slope of that mountain; the rate of its 
travel being somewhat faster than that of the Swiss glaciers. 
In the Bloody Cafion, or Mono Pass from Lyell Meadows, masses 
of slate were met, with a dip towards the axis of the chain, which 
became highly altered about two-thirds from the top of the pass; 
the stratification nearly obliterated, the rock being occasionally 
porphyritic. One mile east of this, and lower down the mountain 
sides, granite 1s again met with and continues to the foot of the 
pass; a finer grained variety than on the western slope of the - 
Sierras, and of a light rose-colour, largely made up of quartz. 
Traces of ancient glaciers were abundant from near the summit 
to the foot of the pass. The rocks, rounded, polished and grooved, 
bear evidences of immense pressure against the walls where the 
cafion is high and shut in; the structure of the rock being changed 
as if by the crushing and closely compacting together of the 
