MTDEN.] 



GEOLOGY — COLORADO EANGE. 29 



from Tiew. We will allude to these terraces agaiu, under tlie liead of 

 glacial eifects, which are very conspicuous everywhere, and nowhere 

 more so than in the valley of the Boulder. 



Kear Boulder City the ridges hug the base of the mountain closely, 

 while between Korth and South Boulders they are worn away, mostly 

 exposing the granites. The terraces, as well as the lower valleys, are 

 literally paved with water- worn bowlders of all sizes. The origin of the 

 agencies which have produced these effects is evident from the 

 fact that as we recede from the mountain the bowlders diminish in 

 size as well as quantity, until far down in the plains they almost 

 entirely disappear. There is no evidence that the superficial deposits 

 are of remoter origin than the immediate mountain-side. At Valmont, 

 in the Boulder Valley, about five miles below Boulder City, there is a 

 singular dike of augitic dolerite, which rises up nearly through the hor- 

 izontal Lignitic strata like a wall. Its strike is about south 67° west. It 

 runs directly up the valley and parallel with it at right angles to the 

 mountain-ranges, and evidently had a much greater extent than at 

 present. It has been uncovered for the most part, and perhaps entirely, 

 by the wearing out of the valley of the Boulder. Originally the igneous 

 material must have been forced up vertically, filling the fissure like a 

 mold; and while the soft yielding sandstones and clays that surrounded 

 it were easily swept away by the eroding agencies, this dike, by the 

 great hardness of the rock, has resisted so that it remains like ver- 

 tical walls. The highest portion is just at Valmont, and is 300 feet above 

 the base, while at either end in a line with the main mass is a raised 

 ridge covered with the fragments of dolerite. At the top it is 30 feet 

 wide, but expands to 50 feet or more at the base, while farther down, 

 where the dike is much broken, it appears to be 150 feet. It undoubtedly 

 varies in width from 50 to 150 feet. The south side of the dike is washed 

 by the Boulder, and is nearly vertical and loose, while the north side is 

 covered with the broken fragments and the sedimentary beds jut up 

 against it without any evidence of much disturbance. From the sum- 

 mit of the highest point the view up and down the valley is very fine, 

 and nowhere in Colorado can be gathered within a single scope of the 

 vision so abundant and so great a variety of the resources of Colorado. 

 The railroads are in operation here, and the broad valley from the 

 mountains down for thirty miles or more is covered with fields of grain 

 and other products of the farm. On the south side in the high 

 hills numerous openings for coal may be seen, and between Boulder 

 Valley and Clear Creek the bulk of the coal of Colorado exists. The 

 valley near the base of the mountains is full fifteen miles in width, 

 but gradually narrows as the long benches that extend down on either 

 side approach each other, and the little branches all unite in one stream.. 

 So far as we can determine from the surface, the dike extends about 

 four miles in a direct line, and from the west end to the east end the 

 strike is about north 30° east. The Lignitic beds incline down the 

 stream, or about west 1° to 3°. About half a mile north of the dike, on 

 the north branch of the Big Boulder, there is a fine exposure of the 

 yellow-gray sandstone, apparently horizontal. Indeed, wherever any 

 of the underlying Sedimentary rocks are exposed, they do not seem to 

 have been disturbed by the forcing up of the igneous matter, a,nd we may 

 therefore conclude that it merely filled an original fissure as a mold. 



I will now pass hastily over the remaining portion of the country to 

 Colorado Springs, presuming that the reader will find a general view of 

 this region, with a pretty clear exposition of the great features of the 



