HATDEN.j 



GEOLOGY COLOEADO RANGE. 25 



semble casts of sea- weeds with fragments of vegetable matter. Then 

 comes black slialy indurated clay, which indicates quiet deposition in 

 moderately deep waters, at least. This is what we have usually denom- 

 inated 1^0. 2, or the Fort Benton group, and sometimes attains a thick- 

 ness of 200 to COO feet, quite homogeneous in character. There is now 

 and then a calcareous layer which is charged with fossils, as Inocera- 

 mus, Ostrea congesta, and other well-marked Cretaceous forms. Above 

 the Dakota group the Cretaceous rocks have very little influence on the 

 scenery further than that the dark saline clays of l^To. 2 and l^o. 4 give 

 the appearance of an arid sterility to the surface, Nos. 2, 3, 4, and 5 

 are usually so soft and yield so readily to the atmospheric agents that, 

 where the mountain-streams emerge from the Dakota group, they flow 

 out into the plains. In some instances they have escaped erosion and 

 occur in a series of low ridges which pass off eastward into the plains 

 like the waves of the sea. This is well shown north of Cache a la 

 Poudre, where the Liguitic group is involved in the uplift and the belt 

 of uplifted ridges is several miles in width, but inclining at very small 

 angles. 'No. 3 Cretaceous is not well defined, yet it has a representation 

 in the yellow and gray limestones and shales or slates. The chalky 

 limestones are often full of the characteristic fossils of No. 3. Along 

 the base of the mountains in Colorado, No. 3 forms rather low, rounded 

 ridges, grass-covered, and not easily studied except where the mountain- 

 streams have cut deep channels directly through them. We can thus 

 trace the continuity complete, so that we find the X3assage from No. 2 to 

 No. 3, and from No. 3 to No. 4, then to No. 5, as gradual as if they 

 were all united in one group w^ithout any possible line of separation. 

 No. 3 varies from 50 to 100 feet in thickness, so far as can be seen, 

 yet the difference in thickness at different points may depend somewhat 

 on the clearness or obscurity of the exposure. 



That all these groups vary in thickness at different localities would 

 be expected, but I doubt very much whether from any exposures along 

 the flanks of the mountains these variations can be determined with any 

 degree of certainty. Nos. 2 and 4, made up as they are of indurated 

 shaly clays, yield more readily to the eroding agents, and No. 3 rises 

 up between them in a low rounded ridge from a few feet to 50 or 100 

 feet in height. Na 2 usually underlies the concave parallel valley 

 or interval between the high sharp ridge composed of No. 1 and the low 

 eroded ridge of No. 3. No. 4 forms the more slightly concave de- 

 pression between Nos. 3 and 5. This series of parallel valleys between 

 the ridges, with a general trend north and south, is a feature peculiar 

 to the flanks of the mountain-ranges, and is best shown, on account of 

 the vast continuous extent, on the eastern side. The intervals are 

 usually softer materials, and have been worn out more or less smoothly 

 by the elements and then grassed over, so that some of the finest farming 

 and grazing lands in the Rocky Mountain' .districts are found here. 

 These valleys are so inclosed that they are protected from the winds 

 and storms, and in consequence old settlers are working their way 

 up from the plains to the immediate base of the mountains in consider- 

 able numbers. As a range for stock these valleys are admirable. The 

 pictorial sections will illustrate what I mean by these parallel valleys. 

 The mountain-streams cut through them at right angles. I have already, 

 in the annual report for 1870, called attention to the wide parallel val- 

 leys north of the railroad between the older rocks and the modern 

 Tertiary or lake-beds. 



In general appearance No. 4 resembles No. 2, yet the latter is more plastic 

 and of a darker color than the former. No. 4 is an indurated clay, some- 



