GEOLOGICAL SUEVEY OF THE TERRITOEIES. 181 



every degree of texture, from a fine, close feldspathic rock with no mica, 

 to a coarse aggregate of quartz and feldspar and fine particles of mica. 

 One of the interesting features of these mountains is the fact that all 

 the little streams find their way through these monoclinal valleys. We 

 see also the main axis of the range, composed of massive granite with a 

 distinct bedding, which is sometimes inclined and sometimes horizontal 

 with the handed gneiss inclining from each side. It seems quite clear 

 that each one of these great ranges of mountains is a grand anticlinal 

 with a massive granite axis, with the gneissic granites inclining from each 

 side in the form of ridges, among which the various streams find their way. 

 The trend of these ranges is in the most cases northwest and southeast, 

 or nearly so. Some of the gneissic rocks in the Platte Valley look like 

 laminated sandstone with a regular dip eighteen to thirty degrees. 

 The tops of the highest ranges are, in some cases, covered very thickly 

 with loose fragments of rocks. 



Passing down from the junction to Denver we have some of the finest 

 examples of jointage structure in the gneissic rocks that I have ever 

 seen; there are two lines of fracture — one with a direction northeast and 

 southwest, the other northwest and southeast, with the lines of bed- 

 ding — making a fine study for the geologist. Some of the beds are thus 

 broken into nearly square blocks, and others in diamond-shaped masses. 



On reaching the base of the mountains the usual ridges of sediment- 

 ary rocks are passed over — red beds, Jurassic, cretaceous, and tertiary. 

 The tertiary beds commence within a mile of the foot of the mountain.s, 

 soon becoming horizontal in their position, and before reaching Denver 

 they are scarcely seen on account of the superficial deposit of drift and 

 alluvial which covers them. 



CHAPTER X. 



TEIP TO MIDDLE PAEK. 



Our route to the Middle Park was through the Berthoud Pass, from 

 the valley of Clear Creek. The range of mountains in which the pas8 

 is located is composed of gneissic rocks — as are all the ranges in the 

 mining districts. The ascent was very steep on the south side, uj) to the 

 region of perpetual snow; but the descent on the north side is quite 

 gradual. 



Great quantities of loose materials from the basis rocks are scattered 

 thickly over the summits, of every variety of the metauiorphic chiss. 

 Most of the peaks are well rounded, and covered with soil and vegeta- 

 tion. Grass and flowers grow far up above the limits of arborescent 

 vegetation. As we ascend, the pines, spruces, and cedars dwindle down 

 in size until they become recumbeat and trail on the ground. Some of 

 the highest peaks are very sharp and covered with loose rocks, as if only 

 the usual atmospheric influences had ever affected them. Their sides 

 are often massive escarpments of rocks down which an infinite quantity 

 of fragments have fallen, making a vast amount of debris at the base. 

 Of course their rocky sides are entirely free from vegetation, and the 

 oxide of iron gives them a rusty reddish appearance. One mountain at 

 the head of Clear Creek is called Red Mountain, from the fact that the 

 rocks have a bright red color in the distance. The evidences of the out- 

 pouring of igneous rocks in this moimtain are very marked; indeed, it 

 may be called an eruptive range. 



