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Show 164 EXPLORATION OF THE CANONS OF THE COLORADO. 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 characteristic 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 producing 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 ones® 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 |
