138 GEOLOGICAL SUEVEY OF THE TEKEITOEIES. 



fined and coitinuous western border, along which their edges are found 

 folded more or less abruply upward, thus forming the zone of uniclinal 

 ridges or hog-backs which lie at the eastern base of the mountains. 

 Along this well-defined line the occurrence of these rather simply com- 

 posed and structured rocks abruptly ceases, and westward for many 

 miles no sandstones, slates, or shales, or other comparatively little- 

 changed and normal sedimentary rocks are to be found. An entirely 

 different class of rocks take their place. First rising either abruptly 

 or in great massive slopes for one to two thousand feet from beneath 

 the lowest of the Triassic sandstones, they form the first great eastern 

 slope of the main range. This slope is not an unbroken north and south 

 line, for all the streams from the range beyond cut great canon-gashes 

 through it, emerging from which they break across the zone of upturned 

 sedimentary ridges and thence out onto the plains. Moreover, between 

 these greater canons, the faces of the more continuous slopes are still 

 cut and broken by almost an infinite number of lesser ravines and 

 gulches which drain down the slope to the first longitudinal valley back 

 to the inner sandstone-ridge, and thence north or south to the adjacent 

 greaier cross-cutting streams. Eising in this manner to a pretty gen- 

 eral uniform level with higher mountain masses here and there, all cut 

 by the caiions into a rugged mountain country, this region extends west 

 for fifteen to thirty miles to the last abrupt rise to the great main crest 

 of the front range, which, in this portion of its course, constitutes the 

 main continental divide, separating the waters of the Pacific from those 

 of the Atlantic, and the topographical characteristics of which were 

 described in Chapter I. The whole of this portion of the great front 

 range, down its less abrupt western slope to the comparatively small 

 and lower-lying sedimentary area of the Middle Park, with nearly all of 

 the elevated country lying south of the park and separating it from the 

 South Park, and forming the connecting link between the Frontand Park 

 ranges, and all of the latter bordering the Middle-Park drainage area 

 on the west, together with the southern extremity of the Medicine-Bow 

 range, which penetrates into the i)ark at its northeast corner, and, 

 finally, a few low, small, and isolated areas in the park itself 5 all this 

 great area, which includes all the grander mountain country, is com- 

 posed of crystalline rocks, schists, gneisses, and granites. 



Disregarding some comparatively small, and, in this connection, wholly 

 unimi3ortant occurrences of undoubted ancient eruptive rocks, (por- 

 phyries, &c.,) as well as some minor granite areas of uncertain eruptive 

 nature, the series as a whole must be regarded as a great system of an- 

 cient sedimentary rocks which have undergone, in greater part, the 

 most profound metamorphism, the result of which, over large areas, has 

 reached that last term of metamorphism, viz, structureless granite. 

 Though, in the region under consideration, the Triassic red sandstones 

 form the oldest recognized sedimentary rocks which rest upon this un- 

 derlying series of crystalline rocks, and thus indicate that they are only 

 at least of Pre-Triassic age, yet further south and west the Potsdam 

 sandstone covers it in large and well-recognized areas, thus demon- 

 strating it to be at least of Pre-Silurian or Archaean age, a conclusion 

 also rendered almost necessary on the independent ground of the extent, 

 uniformity, and comj^leteness of the metamorphism which has affected 

 the mass. For it is no case of local metamorphism, nor one of supposed 

 dependence upon adjacent masses of eruptive rock, nor of the acci- 

 dental presence of mineral waters. The metamorphism is regular, or 

 normal, affecting a great system of bedded rocks of unknown thickness 

 and indefinite extent. Throughout the district examined, this character 



