l minerals_. (2.) _Where these occur, the same sheet of rock may
contain several systems of veins with different ores and gangues._

The great lava plain of Snake River, the Pedrigal country of eastern
Oregon, Northern California and Mexico are without valuable ore
deposits. The same may be said of the Pancake Range and other mountain
chains of igneous rock in Nevada, while the adjacent ranges composed of
sedimentary rocks are rich in ore deposits of various kinds. A still
stronger case is furnished by the Cascade Mountains, which, north of the
California line, are composed almost exclusively of erupted material,
and yet in all this belt, so far as now known, not a single valuable
mine has been opened. In contrast with this is the condition of things
in California, where the Sierra Nevada is composed of metamorphic rocks
which have been shown to be the repositories of vast quantities of gold,
silver, and copper. Cases belonging to this category may be found at
Rosita and Silver Cliff, where the diversity in the ores of the mines
already enumerated can hardly be reconciled with the theory of a common
origin. At Lake City the prevailing porphyry holds the veins of the Ute
and Ulay and the Ocean Wave mines, which are similar, and the Hotchkiss,
the Belle, etc., entirely different.

We have no evidence that any volcanic eruption has drawn its material
from zones or magmas especially rich in metals or their ores, and on the
contrary, volcanic districts, like those mentioned, and regions, such as
the Sandwich Islands, where the greatest, eruptions have taken place,
are poorest in metalliferous deposits.

All the knowledge we have of the subject justifies the inference that
most of the igneous rocks which have been poured out in our Western
Territories are but fused conditions of sediments which form the
substructure of that country. Over the great mineral belt which lies
between the Sierra Nevada and the front range of the Rocky Mountains,
and extends not only across the whole breadth of our territory, but far
into Mexico, the surface was once underlain by a series of Palaeozoic
sedimentary strata not less than twenty to thirty thousand feet in
thickness; and beneath these, at the sides, and doubtless below, were
Archaeun rocks, also metamorphosed sediments. Through these the ores of
the metals were generally though sparsely distributed. In the
convulsions which have in recent times broken up this so long quiet and
stable p