217 



foreground are underlaid with Lignitic strata, oftentimes covered with, 

 a great thickness of drift composed of sands and rounded boulders, 

 varying in size from a small pebble to 10 feet in diameter. The rounded 

 masses are usually quite small. The city of Denver is built on the 

 Lignitic beds, lying nearly or quite horizontal. 



This pictorial section, as well as the one shown on Plate X may be 

 studied with great advantage from several points of view. They pre- 

 sent to the eye the relations of the plains with the great mountain- 

 ranges. We can see how the mountains appear to the traveler, approach- 

 ing them from the eastward, to rise abruptly out of the plains. Denver 

 is 5,197 feet above sea-level, and yet many of the peaks of this great 

 front range, which rises up like a vast massive wall, send their summits 

 above 14,000 feet. The simplicity of the structure of the eastern range, 

 in a general way, is well presented in these sketches. The upturned 

 ridges along the immediate base on the east side, and in some instances 

 on the west side, inclining in opposite directions, show the simple action 

 of a vertical force with very slight tangential movement. In the case of 

 the Laramie range, west of Cheyenne and the Black Hills of Dakota, the 

 full series of sedimentary strata present their uplifted edges on either side 

 of the granite nucleus, so that we cannot resist the conclusion that, i)rior 

 to the elevation, the sedimentary beds extended in unbroken continuity 

 across the area now occupied by the metamorphic central mass ; the 

 missing portions having been removed by erosion during the slow, long- 

 continued process of elevation. 



Although there is apparently so much simplicity in the dynamics 

 of these eastern ranges, the details of structure are very complicated 

 and varied. Perhaps the most important lesson taught by these 

 sketches is the great variety of forms resulting from erosion. The 

 glacial period in the Kocky Mountain region has passed away, but 

 it has left everywhere most wonderful exhibitions of its power. Even 

 the highest peaks have suffered more or less degradation, and it is 

 hardly possible to estimate the amount of material which has been 

 worn from the great central granitic mass. Water and ice are still 

 at work diminishing the height of the loftiest ranges, and yet the 

 forces now in operation are very feeble compared with those which 

 performed the work during the glacial period. Indeed, we may pre- 

 sume that the work of degradation commenced as soon as the area now 

 occupied by the mountain ranges arose above the sea ; and, therefore, 

 the work of destruction of the original forms has been going on for an 

 almost unlimited period of time, dating back nearly or quite to the Car- 

 boniferous period. We may therefore conclude that the present re- 

 markable forms which Mr. Holmes has so accurately and beautifully 

 depicted, are the later results of the handiwork of nature, in her task 

 of earth-sculpture. All through these ranges of mountains are myriads 

 of deep gorges, the channels of the many small streams that, uniting 

 in the plains, form our large rivers. All of these show conclusively 

 that they have been carved out to a greater or less extent by erosion. 

 On either side of the mountain crests are huge amphitheaters, filled 

 with vast quantities of debris of broken rocks, the wearing back, toward 

 the crest, of these channels by the combined action of water and ice. 

 There are also numerous oval areas, usually called parks, varying in size 

 from a few acres to many square miles, which have also been worn out 

 to a great extent by glacial action. Vast quantities of drift, morainal 

 deposits, terminal and lateral, are in most cases found in these mountain 

 valleys, detailed descriptions of which appear in our reports, from time to 

 time. All along the base of the mountains this local deposit is most 



