THE GRAVEL: BETWEEN THE YUBAS. 203 
described as occurring at Smartsville. The bulk of the material consists of 
water-worn pebbles and small boulders, of moderate size and made of 
all the usual varieties of metamorphic and granitic rocks occurring in this 
portion of the Sierra; there are also many pebbles of quartz. Among the 
boulders are some of great dimensions ; but, so far as the writer has observed, 
it is remarkable how near an approach to uniformity of size there is in the 
mass of detrital material taken as a whole. As hardly ever fails to be the 
case, there are occasional beds of tough clay and others of fine sand inter- 
stratified with the gravel, and there is no mistaking the general stratified 
character of the deposit, although portions seem to have been rapidly heaped 
together under the influence of shifting currents. The heliotype reproduc- 
tions of Watkins’s admirable photographs (Plates A — frontispiece — and I) 
show most distinctly the lines of stratification, and the general homogeneous 
character of the boulders, at the North Bloomfield Company’s Malakoff 
Diggings. 
The upper portion of the gravel in this district, as is usually the case in 
the hydraulic mining region of the Sierra, is less compacted together than 
the lower, and can be washed away without previous loosening, or shaking 
up, by means of powder. The top gravel is also of a lighter and more red- 
dish color than the lower portions of the mass, as would naturally be expected, 
the easier access of the atmospheric agencies to the higher parts of the banks 
allowing more or less disintegration of the material and oxidation of the iron. 
The lower strata are, as usual, of a more or less decidedly blue color, and are 
often so solidly compacted as to require a liberal use of powder before the 
hydraulic jet can be brought to bear successfully on the mass. This “ blue 
gravel”’ is of irregular thickness, varying from a few feet to over one hun- 
dred. In shaft No. 1 of the Malakoff Diggings, according to the section given 
by Mr. Bowie, there is mostly blue gravel and “cement” indicated, for the 
whole depth below seventy feet, to the bottom of the shaft, which is about 
220 feet deep. 
To reach the deep gravels in the region between the South and Middle 
Yuba rivers, a considerable number of long and very expensive bed-rock 
tunnels have been run. Mr. Hague gives a tabular statement of the length 
and cost of nine of these. That of the North Bloomfield Company, which 
was sunk with the aid of eight auxiliary shafts, is the longest and most expen- 
sive of them ; in fact, with the exception of the Sutro Tunnel, it is the lar- 
gest work of the kind in the country. It commences in the deep cation of 
