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Show . 200 EXPLORATION OF THE OARONS OF THE COLORADO. ing delight, surveying the stupendous formation through which the Colorado and its tributaries break their way.'' On the 12th of April he obtained another good view across the country to the north, aoo, in his account of the day's journey, he mako!; this remark: "On the north side of the Colorado appeared a short rango of mountains, close to the canon, which had been previously hidden by the intervening plateaus." On the map of the country embraced in this reconnaissance, a group of mountains are indicated, and called, by him, "North Side Mountains "-a name doubtless intended by him as provisional. rrhey are the same as those mentioned by Lieutenant Whipple, and the same that we have described as standing on the bench between the To-ro'-weap Cliffs and the Hurricane Ledge. The Indian name U-in-ka'-rets has been adopted by the people who live in sight of the highest peaks, and so I have adopted the name which will doubtless live among those who use it daily. The most northern of these mountain masses I Lave called Mount Trumbull, the next Mount Logan, and the one standing nearest to the Grand Cafion Mount Emma. · The great mountain masses themselves aro covered with vo1canic cones and groups of volcanic cones are scattered over the benches. Let us se; how these mountains were formed. We have seen that the Uinta Mountains were not thrust up as peaks, but were carved from a vast, rounded block left by a retiring sea, or uplifted from the depths of the ocean, and its present forms aro due to erosion 1 B.u t these are volcanic cones. IIave they, then , been b m'I t up as mount-ams' .we shall see. The beds of sedimentary rocks, on which these mou~tams stand,. ~un un.der the Vermilion Cliffs, to the north, and the beds seen m the VermiliOn Chffs at one timo extended f:a. ·1 · a way t. o tl1 e sout1 1 over thts country and beyond the Grand Cafion. Shales, sandstones and 'lime-stones,. several thousand feet in thickness ' have been was h e d awa'y f rom the summit of all these benches south of the., cliffs. When this denudation commenced, there were no f:1Ults and no ben h ancl stream ran uown from tile north headin()' in tb M k I c CB, p -, ' PI ' ( b c ar- a -gunt and auns a -gunt ateaus, anu found their way into the 0 I . d L o 01a o, am probably U-IN-KA-UET MOUN'!' A LNS. 201 there wore valleys along their courses. Other streams had their sources far away to the south, and came down into the Colorado, and it is probable that thoy also ran through valleys. Then these displacements began; they were not formed suddenly; the rocks wero not flung down during somo great convulsion, but !;ettlod slowly, so that this change in the contour of the surface had no effoc.t on the course of the streams. Thus the downfall of the beds was not faster than the wearing away of the channels, for the displacements by faults and folds has not determined nor modified the direction of the principal streams. As the rocks fell, molten lava was thrust up, not suddenly, nor all at once, but from time to time-now here, now therepouring out a sheet of molten rock in one eruption, and again in another, and this commenced away back in that time before the shales and sandstones seen in the Vermilion Cliffs had been carried away from the benches and plateaus to the south. Doubtless these first floods of lava found their ways into valleys-valleys in that elder time-and covered great beds of these sandstones and shales. When the lavas cooled,· the rocks which they formed were much harder than the sandstones by which they were underlaid, and the beds which formed the surface of the country elsewhere; and as the degradation of this region by rains and rivers continued, the surrounding country was carried away, and the sandstones and shales, protected by the harder beds of basalt, remained; and now mountains stand in such places, doubtless marking the sites of ancient valleys. So the uncovered sandstones wasted away, and the lava-capped beds remained, leaving at first low tables, covered with sheets of basalt. Still, from time to time, new beds of lava were poured out-not over the old beds, usually, but on their borders, increasing their protected area; and, as the surrounding sandstones were still farther carried away, still, pari passu with erosion, came floods of lava, and thus the mountains which remain havo a strangely complex constitution. We may call them eruptive mountains, for, had no eruption occurred, no mountains would have been left; all of the sandstones would have been carried away. But yet the great mass of the material of which the mountains are made is not eruptive matter; tho mountains are great beds of sandstone and shale, covered with blankets of basalt, and, in a. general way, tbe older beds of lava have the higher position on the mountains. 26 COL ' |