278 = GEOLOGY. 
gold was deposited with a thick layer of drift, which was afterwards swept away, leaving the 
gold behind. It would appear that the drift had been re-assorted, or perhaps removed, by cur- 
rents of less strength and extent than those which transported the gold. 
The deposits of the fourth class, called Zacustrine, are found in extensive basin-shaped 
depressions in the surface of the metamorphie rocks. "These depressions have evidently been 
filled with deep and quiet water, from which thick strata of clay, fine sand, and volcanic ashes 
have been deposited upon the auriferous layer at the bottom. The deposits at Georgetown and 
Cement Hill are examples of this class. They are more nearly allied to the alluvial deposits of 
the * flats" than to either of the other forms under which the auriferous drift appears. The 
section of the strata at Mameluke Hill shows that the deposition of clay and volcanic ashes 
from the lake was interrupted, and that a layer of coarse, auriferous gravel was spread out 
over the surface of clay, after which the former conditions were restored. This alternation, or 
a series of auriferous and non-auriferous materials, stratigraphically deposited, forms a well- 
marked difference between the lacustrine deposits and those of the flats. ۱ 
Great changes have been produced in all of these deposits by denudation and erosion during 
and since the elevation of the region to its present level. The old rivers changed their beds, 
lakes were drained, and new streams cut their way through great deposits of coarse drift, 
through lacustrine deposits, and across the ancient river-courses. But the action of the denuding 
streams has not been confined to the superficial deposits, either auriferous or Tertiary ; they 
have eroded great valleys and cafions in the underlying rocks, both of granite, limestone, and 
slate ; all are cut through and traversed by long valleys nearly transverse to the trend of the 
rocks. These valleys of erosion are on a most magnificent scale, and may be regarded as deep 
ravines in a formerly unbroken plateau or slope. Their general form is shown in the section. 
RIVER‏ ---—---- ح 
RIVER CANON— VALLEY OF EROSION IN SLATE. 
On the forks of the American River these eroded valleys are from 1,500 to 3,000 feet deep, 
and the traveller who desires to cross from one bank to the opposite side must wind in a zig-Zag 
line down one side and in a similar manner up the other; traversing a distance, in most cases, 
of nearly three miles, while in a direct line it may be but little more than one mile from one 
bank to the other. All this erosion has taken place since the original deposition of the gold; 
and it is probable that the gold of the streams is derived from the original deposits ین‎ 
drift rather than from the action of the rivers upon the veins, although a very considerable 
quantity of gold must have been liberated from the veins by their action. The great currents, 
or floods, which produced the drift were much more general and wide-spread in their action, 
and appear to have abraded the whole surface rather than mere lines or channels. 
All the observations upon the auriferous deposits sustain the conclusion arrived at by 
Roderick I. Murchison, after his explorations in the Urals—that the formation of gold " gue 
gically speaking, very modern ; that it is one of the most recently formed metals ; the r " 
being probably impregnated with it after the Miocene period, and but ‘‘a short time before the 
Sir 
