Sie 
THE MOUNTAINS OF COLORADO. ; 69 
used at Denver for building purposes, which Hayden regards as 
Carboniferous. Although ripple-marked, I observed no. fossils. 
Both of these deposits are highly metamorphosed and the strata 
stand nearly vertical. 
Next succeeds a vast series of gneissoidal rocks in which feld- 
spar and mica are the predominant minerals. These rocks every- 
where show lines of bedding, but they have been plicated, shat- 
tered and tilted up at all angles, and at the same time are cut by 
numerous divisional planes. The metamorphism of the mass is 
so complete as to have obliterated all traces of fossils and to have 
changed the mechanical structure of the rocks themselves. Per- 
haps there is no region on the continent where the action of i Apne 
ous causes is displayed on so grand a scale as here. 
The true granites are only seen along the axes of elevation. 
They play an important part in the structure of the region, con- 
stituting, as it were, its framework. 
In the’ Rocky Mountain system is probably represented the 
whole assemblage of formations from the Azoic up to and includ- 
ing the Jurassic, but so thorough has been the processes of meta- 
morphism, at least on the Atlantic slope, that it is impossible to 
recognize subordinate groups. On the western slope, Fremont 
long ago recognized rocks with organic remains, which he referred 
to the Odlite, which is a member of the Jurassic. The investiga- 
tions of Whitney in California have settled this question—that it 
was at the close of the Jurassic epoch that this vast assemblage of 
formations was metamorphosed and folded into great ridges with 
their intervening valleys. The eruptive rocks accompanying this 
upheaval were for the most part granites, probably in a pasty con- 
dition, as in this association there are no traces of volcanic prod- 
ucts. 
Vers Poenomena.— It was at this time that the granites and 
metamorphic rocks became impregnated with the precious metals, 
such as gold and silver, which are found concentrated in veins and 
fissures. At a subsequent date— during the earlier Tertiary Pe- 
riod—a series of vof@fhic vents were formed along the line of pre- 
vious disturbance, from which were poured forth a series of igneous 
products, such as basalts, lavas, etc. These also became impreg- 
nated with the precious metals, of which the famous Comstock lode 
in Nevada, as shown by Richthofen, is a notable example. Thus, 
