216 



except in this instance. The inclination of these sandstone ridge-s 

 varies much in different localities. Sometimes they stand nearly ver- 

 tical. They may be said to incline at all angles from 5° to 80° from the 

 granitic mass. It will be seen that these sketches illustrate a very im- 

 portant point, which has been often repeated in my former reports, the 

 rising of the great mountain-ranges suddenly or abruptly out of the 

 plains. In the foreground we have the Lignitic beds inclining at a very 

 small angle, within a short distance from the granites, varying from a 

 half mile to two miles in width, seldom more. We may infer, there- 

 fore, that the force that elevated the mountain-range acted nearly or 

 quite vertically. Whenever the inner ridges stand at a high angle,, 

 the beds haA^e been undoubtedly broken off abruptly close to the granite 

 foot-hills, as is well shown at Boulder Creek, and at Pike's Peak in the 

 " Garden of the Gods." The illustration of this vertical uplift of the 

 sedimentary beds, by which they seem to have been broken off abruptly, 

 forming a right augle, as it were, is very common along the base 

 of either side of the eastern range of the Eocky Mountains. In some 

 instances a nucleus of granite will be surrounded with a narrow belt 

 of nearly vertical beds of sedimentary rock, and within a few hundred 

 feet the same vertical strata will lie in a nearly horizontal position. 

 Some of the most important coal-beds in Colorado are opened within 

 a mile east of the granite nucleus in the drainage of the Boulder 

 Creeks, the strata of the coal group inclining at angles of not more 

 than 10° to 15°, and very soon flattening down to a nearly or quite hori- 

 zontal position still farther to the eastward. 



Let us examine the sketch from Denver. To the west, about ten miles, 

 are two quite remarkable table-mountains capped with basalt. Under- 

 neath their basaltic caps there is a great thickness of the Lignitic beds, 

 evidently protected from erosion by the hard bed of basalt over 

 them. The valley of Clear Creek separates the two. That they origin- 

 ally formed one bed and spread over a much larger area than at present, 

 seems probable. Between these table-mountains and the granites, the 

 distance is not over a mile in a straight line, and yet there is a thick- 

 ness of 1,500 feet or more of Lignitic beds, with an important coal-seam, 

 with a series of Cretaceous, Jurassic, and red sandstones, or Triassic, 

 in regular order to the granites. These mountains, with the geology in 

 the vicinity, were described by me in the third annual report of the sur- 

 vey, 1869, and much more in detail, with illustrations by Mr. Marvine, 

 in the seventh annual report of the survey for 1873. From Denver, 

 southward of the Platte Canon, the ridges are well shown in the sketch 

 just under the dotted line. At a distance, there would appear to be 

 not more than one main ridge, but there are usually several of them, 

 with beautiful valleys between. There are many fine farms among these 

 ridges, and the settlers are very numerous at this time. South of the 

 Platte Caiion there is a narrow belt of the red sandstones extending for 

 about five miles that have been worn into most remarkable picturesque 

 forms, presenting as great a variety, even, as the celebrated "Garden 

 of the Gods," at Colorado Springs. The red sandstones are protected 

 from the plains by a continuous ridge of the Cretaceous sandstones of the 

 Dakota group, while, between that ridge and the granites, there is a 

 belt between a fourth and half a mile in width, in which the red sand- 

 stones present a confusion of broken ridges, with their sharp edges pro- 

 jecting above the level, grassy surface, from a few feet to 150 feet in 

 height, with an inclination from the granite mass of 45° to 50° to the 

 east. For this entire distance the Eed beds, or Triassic, (?) as they have 

 been usually termed, rest on the granitic rocks. The rolling plains in the 



