-I GEOLOGY FROM ELK MOUNTAINS TO MIDDLE PAEK. 79 



to 100 or 150 feet. In No. 5 are a great abundance of fossils of the 

 genera Inoceramus, Baculites, Ammonites, &c. 



The surface of the enth-e country is exceedingly rugged, and the high 

 ridges, underlaid with ISTo. 5 or No. 1, contrast curiously by the lux- 

 uriance of vegetation with the barren, sage-covered surface of the black 

 plastic clays of Nos. 2 and 4. The high ridges of the divide are 

 covered with vesicular basalt, much of which is spongy, like lava. The 

 dikes have a trend about 20'^ south of east, and as they are very numer- 

 ous and are nearly parallel, resemble waves of the sea as they extend 

 along the surface in more or less elevated ridges. These basaltic 

 ridges are sometimes continuous for several miles, but they usually 

 break up into fragments, yet still preserving about tbe same trend. As 

 we travel from Eagle Eiver to the valley of the Blue, the surface is 

 covered to a greater or less extent with loose fragments of modern 

 basalt, varying much in texture and color. The country reminded me 

 very much of portions of the Yellowstone Valley in Montana. As we 

 descend into the valley of the Blue, below the junction of the Grand, 

 we can see that the high ridges on either side are capped with basalt, 

 but underneath are the black clays of the Cretaceous. 



A great thickness of the sedimentary beds cover the country, but 

 they are literally riddled with dikes, and the strata are thrown into 

 complete chaos. On the north side of the Blue Eiver, about fifteen 

 miles below the junction of the Grand, are several long mesa-like ridges 

 that rise 1,000 to 2,000 feet above the river-bottom, and apparently 

 incline from an extension of the Blue Eiver range along the west side 

 of Middle Park. 



These mesas present a structure too complicated for our limited time, 

 but the entire mass seems to have been lifted up nearly vertically, pro- 

 ducing great faults, so that on the south side the variegated beds which 

 lie beneath the Cretaceous group were tipped up at the base in a verti- 

 cal position, and sometimes 15° to 20<^ past a vertical. 



These mesas, capped with basalt, slope southwest 5° to 15"^. In the 

 sides of these mesas several beds of basalt are shown, varying from 4 

 to 40 feet in thickness, with a layer of volcanic tuff intervening. 



To work out the complicated structure of this most interesting region 

 would require the diligent labor of a season, and I can in this report 

 only notice it in general terms. There are, however, most abundant 

 illustrations of the action of the two forces, one of which, long contin- 

 ued and uniform, produced the anticlinals which are continually inter- 

 rupted or thrown into confusion by the eruption of volcanic matter. 

 Along the Grand and Blue Elvers, on the west of the Middle Park, are 

 three quite important caiions, the Lower, Middle, and Upper. The Lower 

 CaQon is formed by the passage of the Blue Eiver at right-angles through 

 a high ridge of feldspathic gneiss, with walls 1,200 feet high. There 

 seem to be a number of these granite uplifts, extending with a nearly 

 north and south trend across the park. The most western one rises 

 about 1,200 to 1,500 feet above the bed of the river. From either side 

 of these granite nuclei the sedimentary beds incline at various angles. 

 The Middle CaSon is about three miles below the junction of the Grand 

 Eiver with the Blue, and is much the most extensiv^e. The Lower CaQon 

 is only about one- third of a mile in length, while the Middle Canon is 

 full three miles long. 



The anticlinal ridge, as it might be called, is really an extension north- 

 ward of the Blue Eiver range, and forms the west rim of the Middle 

 Park. The slopes on the east side are all gentle, and are mostly com- 

 posed of the quartzites of the Dakota group. So far as I could ascer- 



