126 GEOLOGICAL SURVEY OF THE TERRITORIES. 



gently down, while the west side is abrupt; and on the western portion 

 vice verm. But if we take the ridges siuglj^ and examine them, we shall 

 find in most cases that the aggregate trend is nearly northwest and south- 

 east. The consequence is, that as we pass along under the eastern flanks 

 of the mountain from north to south, these minor ranges or ridges pre- 

 sent a sort of '■'•en echelon''' appearance; that is, they run out one after 

 the other in the prairies, preserving the nearly north and south course 

 of the entire system. Not unfrequently a group or several of these 

 ridges will run out at the same time, forming a huge notch in the main 

 range. This notch in most cases forms a vast depression with a great 

 number of side depressions or rifts in the mountains, which give birth to 

 a water system of greater or less extent. Such, for example, is the notch 

 at Cache a la Poudre, Colorado City, Caiion City, on the Arkansas Eiver, 

 and other localities. If we were to examine the excellent topographical 

 maps issued by the War Department, which are beyond comparison the 

 most correct and most scientific of our Eocky Mountain region in exist- 

 ence, we should at once note the tendency of all the minor ranges, with 

 a continued line of fracture and a granitic nucleus, to a southeast and 

 northwest trend ; sometimes it is nearly north and south, and then these 

 ranges pass out or come to an end without producing any marked in- 

 fluence on the topograph}^, except, perhaps, some little stream will flow 

 down into the plain through the mouoclinal rift. But when several of 

 these minor ranges come to an end together, an abrupt jog of several 

 miles toward the west is caused. Then frequently as the range dies out, 

 a local anticlinal or a semi-quaquaversal dip is given to the sedimentary 

 beds. Between the notches or breaks in the mountains, the belt of 

 ridges or ''hog-backs" becomes very narrow, sometimes even hardly 

 visible, and sometimes entirely concealed by superficial deposits. But 

 at these breaks the series of ridges split up and spread out so as to cover 

 an area from half a mile to ten or fifteen miles in width. It is in these 

 localities that the complete geological structure of the country can be 

 studied in detail. I do not know of any portion of the West where there 

 is so much variety displayed in the geology as within a space of ten 

 miles square around Colorado City. Nearly all the elements of geological 

 study revealed in the Eocky Mountains are shown on a unique scale in 

 this locality. The same may be said, though in a less degree, of the 

 valley of the Arkansas as it emerges from the mountains near Canon 

 City. I am inclined to believe that it is only in these localities that 

 rocks older than the triassic or red beds are shown along the eastern 

 flanks of the mountains south of Cheyenne. I have looked in vain for 

 a single exj)osure of well-defined paleozoic strata from Big Thompson to 

 Colorado City, a distance of over one hundred miles. I am now con- 

 vinced that in the north, the paleozoic rocks are often concealed for long 

 distances, although I have usually represented them by colors on a geolo- 

 gical map by a continuous band along the mountains. That they exist 

 continuously along the eastern margins in Colorado and New Mexico I 

 cannot doubt, biit only at these specially favored localities do they 

 appear from beneath the triassic or red beds. They are, however, far 

 more frequently exposed further northward, and I think much more 

 largely developed. 



Between Big and Little Thompson Creeks the ridges are very numer- 

 ous and bold, and it would seem as if the massive fine-grained sandstones 

 pi'edominated, for they cap all the ridges, and the broken masses, often 

 of large size, are scattered in great profusion everywhere. In one valley 

 the abrupt side, which was composed of red sandstone, presented an 



