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Show ANTBCEOEN'l' AND OONSEQUENl' VALLEYS. 163 moisture, and a greater quantity of rain falls on them, or in their vicinity. The region of country adjacent to the mountains receives a portion of thh; extra rain-fall, so that this dynamic agency increases from tho plains to the summits of th.e mountains, probably in some direct ratio. This increase of tho eroding agoncy, and the greater exposure of the soft beds, probably accounts for tho fact that the lowest country is at tho foot of tho m nntains. 'l,hero is a limit to the effect of those conditions, for it should bo observed no valley can be eroded below the lovol of the principal stream, which caiTies away the products of its surface degradation; and where tho floor of such a valley has been cut down nearly to the level of such a stream, it receives the debris of the adjacent cliffs and mountains, and in this way the rocks composing the floor arc usual1y masked, to a greater or lesser extent. The same topographic facts, under Hke conditions, aro fonnd on the eastern slope of the Rocky Mountains, in Colorado 'rerritory, and th~ valleys which run into the South Platte from the south, between the hog· backs, are lower than the mesas and plateaus farther away from the mount .. ains, but not lower than the flood plain of the river. I have endeavored above to explain tho relation of the valleys of the Uinta Mountains to the stratigraphy, or structural geology, of the region, and, further, to state the conclusion reached, that the drainage was established antecedent to the corrugation or displacement of the beds by fault.ing and folding. I propose to call such valleys, including the orders and varieties before mentioned, antecedent valleys. In other parts of the mountain region of the west, valleys are found having directions dependent on corrugation. I propo!:!e to call these consequent valleys. Such valleys have been observed only in limiteu areas, au(t have not been thoroughly studied, ana I omit further diSC\lSSion of thorn. In tho great metamorphic belt extending through tho 'ferritory of 101- orado, comprising the Rocky Mountain chain of this Territory, the structural geology is exceedingly complex, while tho drainage is comparatively simple, and only to a limited extent does it seem to be governed by geological structure. '1.1he conclusions to which I arrived were that the present drainaO'e was established in rocks now carried away from the higher regions, but b • still seen to be turned up against the flanks of most of the ranges. |
