Reports of explorations and surveys, to ascertain the most practicable and economical route for a railroad from the Mississippi River to the Pacific Ocean: Vol. 5, Pt. 2

GEOLOGY.
formed by the decomposition of the rock. The spaces between these vertical crags of limestone
are filled in with earth, clay, and gravel, and large fragments of slate. Gold is found in a
stiff, blue clay, at the bottom and sides of the spaces.
All the phenomena point to a most remarkable and rapid solution of the surface of the limestone, and it appears to have taken place after the surface was covered with the auriferous
deposits. Such a surface could never be formed by denudation or the wearing action of currents;
it is totally different, and the irregular and ragged, yet smooth, surface of the blocks shows the
action of solvents, and is not the result of attrition. The imbedded pear-shaped masses of limestone, with the points downwards, show the action of solvents subsequent to the deposition of
the gravel, for they are supported by it. If the solution of the rock has thus been subsequent
to the deposition of the auriferous clay and gravel, these accumulations must have sank downwards by their own weight as rapidly as the underlying rock was removed. This is indicated
by the fact that gold is found imbedded in blue clay on the 8ides of some of the vertical masses
of limestone, while the adjoining materials, at that level, are merely coarse gravel and fragments
of slate. A portion of the clay, containing the gold, would thus appear to have adhered to the
sides of the fissure, while the central portions were forced downwards by the weight of the
superincumbent materials. I am unable to account for these conditions in any other way. It
is not possible to determine the nature of the solvent with certainty, but it is most probable that
it was acidulated water formed by the decomposition of pyrites in the drift and soil, or, possibly,
water charged with carbonic acid alone. The whole surface may have been submerged for a
long time, forming the bottom of a shallow lake or marsh. This is indicated by the uniform
deposition of the clay and other materials over a wide area. It is, however, quite possible that
all the decomposition has been accomplished by percolating water alone, since the surface became
nearly or quite dry. An enormous quantity of limestone has been removed, the fissures in many
places being over twenty-five feet deep, and the solution must have drained away underground
towards the river, or formed deposits of gypsum or travertin in the vicinity. The general
appearance of the cross-section of a broad excavation among these projecting crags is shown in
the figure, the trend and inclination of the limestone being retained by the remaining masses.
It is probable that at some points the spaces between the masses of limestone extend downwards
for a very great distance, and may have been worn by subterranean currents far beyond the
reach of ordinary mining operations. The gold obtained from these deposits is coarse, but
seldom exceeds the size of peas or beans.1
A part of the blue clay from this vicinity is exceedingly rich in fine gold, but its tenacity and
toughness prevents the gold from being readily washed out. The clay is, therefore, raised to
the surface and spread out to dry in the sun, and is afterwards pulverized and then washed.
Enormous teeth, both of the Mammoth and Mastodon, have been obtained here in considerable
numbers, but I was disappointed to find that they had all been burned in the recent conflagration. It is said that somne of them were imbedded in the jaw. At another fiat, several miles beyond,
a large tusk was exhumed, but, being exposed to the weather in front of a miner's cabin, it
soon fell to pieces, and was lost.
We descended long and very steep hills to the level of the Stanislaus River and crossed by
Abbey's Ferry. Granite was observed. The slopes of the River canon are wooded with pines
and oaks. Granite and white limestone were observed after crossing; the limestone trending
1 Sir R. I Murchison gives a figure of an eroded and irregular surface of limestone ia the Urals, from which auriferous
alluvia had been removed by mining. This surface was, perhaps, formed in the same manner as the irregular surface of the
limestone in California. —Russia in Europe and the Urals, i, p. 487. Siluria, p. 442.
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