164 EXPLORATION OF THE CANONS OF THE COLOEADO. 
A part of the district in which my observations were made has since 
been much more thoroughly studied by Mr. Archibald R. Marvine, one of 
the geologists of the First Division of the " Geological and Geographical 
Survey of the Territories." In his report of June 19, 1874, he says: 
"Three causes combine to render the rapid study of the stratigraphy 
of the archsean rocks difficult and its results uncertain: First, their structure 
is not only often complex, but obscure, the evidence of it being at times 
nearly or wholly obliterated by the metamorphism, and often over large 
areas very difficult to find; second, this metamorphism renders lithological 
characters inconstant, so that a stratum that at one point may be character 
istic among its neighbors, may, at another, become like them, or all may 
change so as to retain none of their geological features, becoming again 
like other series, so that lithological resemblances cannot often be taken as 
a guide to follow, and may even become misleading; third, the erosion pro 
ducing the present surface features of the mountain region had the direction 
of its action determined by movements of the surface which were not 
closely connected with the extended plications of its rocks; and, moreover, 
since this erosion has not long been acting among these rocks, there appears 
no well defined connection between the topography and the structural 
geology. The ancient erosion gradually wore down the mass to the surface 
of the sea, and while previously to this it was no doubt directed by the 
structure, yet the mass was finally leveled off irrespective of structure or 
relative hardness of its beds by the encroaching ocean, which worked over 
its ruins and laid them down upon the smoothed surface in the form of the 
Triassic and other beds. The recent great uplift, while it probably added 
new plications to the accumulated plications of the past in the ancient 
rocks, was quite simple with respect to their total plication, and left the 
upper Triassic and other sedimentary beds comparatively simply structured, 
they having been affected alone by the later movements. 
"As the mass appeared above the sea and surface erosion once more 
commenced, but which now acts upon the recent rocks covering probably 
in greater part the complex underlying rocks, it was directed off from the 
line of greater uplift down the long slopes of the rising continent to the 
retiring sea. The channels of drainage started were directed solely by the 
