SURVEY OF THE COLORADO OF THE WEST. 
21 
of lava; vet the mountains themselves are not principally composed of 
lava, so that they can be called eruptive mountains only in the sense 
that had there been no eruption there would have been no mountains, 
but the plateau would have been degraded to a comparatively uniform 
plain. 
The general statement may be made that all the eruptive mountains 
in the region which we have surveyed exhibit like facts. 
Besides the mountain-masses due to eruptive agencies, and composed 
in part of eruptive matter, there are many later floods of basalt found 
throughout the country, spread as great protecting sheets over sedi- 
mentary beds, or elsewhere refilling valleys or canons; and, with few 
exceptions, all of the floods of basalt are crowned with cinder-cones. 
The faulting and folding, of which mention has heretofore been made, 
independently of the eruptive matter Avhich is found so widely dis- 
tributed, reveals the fact that the sedimentary beds of this country 
have been fractured on an extensive scale, and we are able to trace 
great lines of fracture in tracing the faults and folds. The relation of 
the eruptive beds and volcanic cones in geographic position to the great 
fractures that have been discovered in the faults has been a subject of 
much study, and interesting results have been obtained. The oldest 
beds of the series of eruptive rocks above mentioned overlie Eocene- 
Tertiary formations, and the succession of later beds, which have been 
poured out pari passu with degradation, are found to overlie sedimentary 
beds of earlier geological formation. So, in general terms, it may be 
stated that the later beds of igneous origin are associated with the earlier 
beds of aqueous origin. Thus, floods of basalt have run down into 
valleys deeply eroded in Carboniferous strata at a period so late that 
sufficient time has not elapsed since their cooling for atmospheric agen- 
cies to have any appreciable effect upon them. Nor are they even 
lichened. It is more than probable that some of these have been poured 
out within the last two or three centuries. 
Besides this great period of eruption, thus briefly mentioned, there 
are two other eras of eruption revealed to the geologist in this region. 
It has already been stated that the Carboniferous beds are deposited 
unconformably upon the underlying Silurian and Devonian beds, and 
the period of erosion intervening between the disposition of these two 
unconformable series was also a period of eruption, for again we find 
faults, dikes, and beds of trap in the rocks of greater age, which do not 
penetrate the beds of Carboniferous age. 
The metamorphic crystalline schists revealed in the section along 
the Grand Canon are seen to underlie Devonian and Silurian rocks un- 
conformably ; and here again, between the junction of the two, we find 
another period of eruption, for the schists are traversed by dikes and 
beds of granite, but which do not penetrate the overlying Devonian and 
Silurian rocks. 
