306 GEOLOGICAL SURVEY OF THE TERRITORIES. 



appearance that is imparted to any landscaije by tbe predominance of 

 arclieean formations. Westward from the edge of the great plains the 

 mountains rise abruptly, particularly so in the vicinity of Pike's Peak, 

 the summit of which is more than 8,000 feet above the valley. In the 

 pure atmosphere of that region, th^e deception in distance allows the 

 traveler to be even more strongly impressed with the grandeur of the 

 scenery that has given the '• Pike's Peak country" so much of its justly- 

 deserved celebrity. Along the eastern border of these mountains, 

 ■where they gradually taper ofi' into the plains, stretched for many miles 

 before them, the sedimentary beds reclining against the granitic mount- 

 ains have at times been greatly disturbed, and now form at several 

 points verj^ grotesque groups along their base. 



Going westward from the border of this Front range, more into the 

 interior of the mountains, unmistakable cones appear, denoting former 

 volcanic activity, changing with their introduction the general aspect 

 of the country. Instead of densely-wooded hills, covered either by 

 living or dead timber, areas of grazing-lands are frequently found, fre- 

 quentl}" also marshes. In many cases the prommeut cone-shaped peaks, 

 the probable points of volcanic outflow, present a more desolate appear- 

 ance, being covered with innumerable fragments of rock, rarely large, 

 but sufficiently so to prevent any extensive growth of vegetation. A 

 variation from this general character of the country occurs along Oil 

 Creek, where a comparatively low tract of country extends northward 

 for the distance of about twelve miles, with a width varying from two 

 to four miles. To the west of this little valley the granitic rocks are no 

 longer the predominating, for the volcanic increase so largely in bulk 

 and extent as to change tbe face of the country. High, grassy plateaus 

 appear, differing in vegetation to such an extent from the older rocks 

 tbat may be outcropping that this difference alone may frequently fur- 

 nish useful hints to the geologist regarding the distribution of the 

 formation. Quite frequently the creeks have worn deep, rugged ravines, 

 although it would undoubtedly be erroneous to attribute all the caiions 

 and gorges to erosion exclusively. Farther toward the northwest, the 

 character of the country again changes ; it becomes more rolling, sup- 

 plied with frequent cones ; the valleys are broader, less deep. 



In the western portion of section a, the mountains are no longer so 

 high, and their slopes are more gradual on the northern side, although 

 often quite rugged on the opposite; the influence pf comparatively little 

 disturbed sedimentary formations is perceptible. Bordering upon the 

 Arkansas, the hills of this western portion are less massive — less so than 

 those farther east — owing to the lower relative and absolute altitude 

 and consequently smaller depth of the caiions. 



Granite forms the heaviest masses of section a, although not covering 

 the greatest area. On the western part of the section it occurs as the 

 main rock, presenting a dark-red or brown color, showing but little vari- 

 ation in the character of its texture and structure, and usually weath- 

 ering in heavy bowlders ; it frequently forms bold and prominent faces 

 on the mountains. Toward stations 89, 90, and 91, it is mostly covered 

 by the overflowing trachorheites, outcropping only in the deeper canons 

 and gorges that are in a great part due to erosion. Owing to its coarse 

 texture, it readily decomposes, but, dependent upon the more or less 

 accidental percentage of its constituting minerals, does not decompose 

 uniformly throughout. At station 95, the granite becomes very coarse- 

 grained, so much so as to show a perfect separation of the three min- 

 erals, feldspar, quartz, and mica ; only tbe latter, however, follows its 

 inclination to crystallize. At this locality, the feldspar.(orthoclase) is of 



