HATDEN.J 



GEOLOGY COLORADO RANGE. 23 



pebbles, antl other Metamorpbic rocks, witb a coarse qnartzitic sand 

 as a cement. There -were many deep depressions, which are filled up 

 in a remarkable manner. These tongues or ridges of granite extend 

 down from the main rauge in many places, but seldom so far as to 

 break the regularity of the upheaved sedimentary ridges. Between Big 

 ThomiDson and Cache a la Poudre there are quite wide grassy intervals 

 between the ridges, which produce excellent grazicg for cattle. These 

 ridges are well defined ; there are usually about two principal ones of 

 the red group, and one very high and prominent ridge, composed of i^o. 

 1, underlaid i^robably with the Jurassic group. The Dakota group 

 ridge is immensely developed all the way to Cache a la Poudre. On 

 the inside of the ridge vast blocks of quartzite and pudding-stone, 20 

 feet cube, have fallen down into the vallej" or lie thickly scattered on 

 the sides of the ridge. At Spring Canon, nine miles north of Big 

 Thompson Station, there is a splendid section of the entire sedimentary 

 group. The little streams seem to have cut their channels direct from 

 the mountain-sides through the series of ridges at right angles. The 

 Dakota group is at least 250 feet thick, and is composed of beds of fine- 

 grained sandstone or quartzites, which are much used for building pur- 

 poses. So much has already been written in regard to the Cretaceous 

 group, as shown south of the railroad, that I do not know that I can 

 add anything new in this chapter. The very minute and accurate sec- 

 tions of Mr. Marvine will make the succession of the beds, as well as 

 the relative thickness from point to point, perfectly clear. Between 

 Cache a la Poudre and Big Thompson, the ridge composed of No. 1, 

 or the Dakota group, is most conspicuous, forming a peculiarly sym- 

 metrical roof. This ridge is higher and more uniform than any of the 

 others on either side of it, due probably to the fact that the texture 

 of the rocks of the Dakota group is so much more compact and resists 

 the wear of the atmosphere more effectually. The slope of the roof is 

 about 20°, and although much of the surface is bare, the upper portion is 

 covered with scattering cedars of a stunted growth. At irregular but 

 short intervals inverted conical notches occur, produced by the little 

 streams, which have worn their way, to a greater or less depth, directly 

 through the ridge. The more important streams have cut deep channels 

 from the mountains through into the open plains, but in the intervals are 

 numerous depressions, like those shown in the illustrations, that indicate 

 the erosion of temporary streams, thus giving a wavy outline to the 

 outcropping edge of the ridge. We may say here in this connection 

 that the Dakota grouji is one of the most widely-distributed forma- 

 tions in the West. To attempt to describe the variations in structure 

 from point to point would be an almost endless task, and yet, when care- 

 fully studied in one locality by the geologist he never fails to detect its 

 presence at other points where it exists, if exposed. All over the great 

 middle belt of the West, so far as I have observed it, between the paral- 

 els 47° and 34"^ and the meridians 97° and 114°, it maintains enough 

 of its peculiar lithological character to be readily detected, and thus, 

 although in most instances destitute of, or containing very imperfect 

 organic remains, it forms a permanent basement-floor for the great Cre- 

 taceous formation of the West, as well as a most important datum line 

 for determining the age of the rocks above and below. The numerous 

 species of plants, with a few invertebrate fossils, which have been found 

 on the Missouri Eiver and in Nebraska and Kansas, have fixed the age 

 of the group, so that we believe it passes beneath all the more modern 

 beds from their points of appearance at the East to their exposure along 

 the flanks of the mountains. The eastern portions of this group are 



