406 GEOLOGICAL EXCURSION TO THE KOCKY MOUNTAINS. 



On the eastern flanks of the Colorado or Front range the Paleozoic 

 and earlier Bfesozoio beds appear only in disconnected patches, being 

 covered in the intermediate portions by beds deposited during the late 

 Jurassic, post-Cretaceous, and Tertiary transgressions. A narrow- 

 zone of sharply-upturned beds, mostly of Mesozoie age, is generally 

 found at the immediate base of the mountains, the harder beds form- 

 ing-narrow monoelinal ridges called •• Hogbacks," which are separated 

 from the main mountain slopes by longitudinal valleys eroded out of 

 the softer beds of the series. These sharply-upturned beds, whose an- 

 gles of dip are as high as 00° or 70°, change sharply to an approxi- 

 mately horizontal position in very short distances east of the moun- 

 tain foot. In some cases the outer or upper beds of the upturned series 

 stand at a steeper angle than those below, or nearer the Archean base, 

 producing thus a partial fan structure. The overlying Tertiary beds 

 sometimes partake to a limited extent in the upward curve of the un- 

 derlying Mesozoic strata, thus evidencing a comparatively recent 

 movement of uplift of the mountain mass, or a sinking of the plain area. 



The coal-bearing beds of the Laramie Cretaceous, which were 

 deposited as the ocean waters were finally leaving the western United 

 States, spread not only over all the Great Plain areas around the 

 mountain groups but also over many of its interior valleys. These beds 

 being the first of the series to be acted on by the forces of erosion, have 

 been removed from considerable areas t hat they once covered, especially 

 along the eastern portion of the Great plains and the southern por- 

 tion of the Colorado basin. In other areas they have been deeply 

 buried beneath succeeding deposits of Tertiary age. In spite of these 

 facts, the available coal-bearing areas that still remain are of enormous 

 extent, and already play an important part in the industry of the 

 region. The Census reports for 1890 show that in that year the coal 

 mines of Colorado produced 2,300,000 tons of coal and gave employ- 

 ment to 4,04:5 persons. The coal varies in character from a rather 

 light, dry, porous coal, with high percentage of water and of volatile 

 matter, to fairly dense caking or coking coal. The latter coals are 

 generally found to the south and west, in regions where eruptive rocks 

 abound. In the southwestern Elk mountains, moreover, are beds of 

 excellent anthracite. 



The aggregate thickness of the various geological series has, as a 

 rule, very materially decreased from that shown in the Wasatch region. 

 A greater mass of Archean is shown becanso it has been more deeply 

 eroded. The Ouray beds (assumed to be Algonkian) of the San Juan 

 mountains are over 10,000 feet thick. On the other hand, but a few 

 hundred feet of Paleozoic beds below the Carboniferous are found, the 

 lower members of the Cambrian and the entire Devonian being appar- 

 ently unrepresented. The Carboniferous series is the most generously 

 developed of any in the Paleozoic group, but this averages only about 



