n^YDEN.l GEOLOGY— COLORADO RANGE. 33 



although the air, rain, and snow may have done much to give the 

 monuments their i^resent forms. The greater portion of the erosion 

 must date back into the past, at least to the Post-Pliocene period. It is 

 very probable that water had much to do with the formation of these 

 monuments at a time when there was a far greater supply here than 

 there is at this i)eriod. The entire surface of the country must have 

 been on a level with these caps at least, and probably much higher. 

 The caps themselves are nothing more than concretions mostly rounded 

 and flattened, while intervening materials have been slowly worn out 

 and carried away. There is another form of erosion which is character- 

 istic of this formation, and that is the mesas, or table-buttes, which are 

 scattered thickly over a broad belt east of the Denver and Eio Grande 

 Eailroad. Most of these mesas are capped with a purplish porphyritic 

 basalt which originally flowed over the surface, doubtless covering a 

 broad area. These buttes, with their flat summits, were also carved 

 out of the horizontal strata, and vary in height from 100 to 150 feet. 

 The rocks composing the " mesas" are mostly of a finer material ; and 

 above the coarse sandstones of the monuments some of these mesas 

 are nearly round, others square, others oblong. On the east side of 

 Monument Creek and near the divide they assume curious castellated 

 forms, so that they look in the distance like the ruins of old castles. 

 These may be seen in considerable numbers in the distant plains as far 

 south as Colorado Springs. There is a large area in the plains east of the 

 Denver and Eio Grande Eailroad line, from Denver to the Arkansas Eiver, 

 which is not well known to the geologist as yet. The pine-covered ridge 

 from which so many streams rise flowing northward into the South Platte 

 and south into the Arkansas, forming what is located on the maps as 

 the Bijou Basin, must be underlaid by rocks which I regard as belong- 

 ing to the Monument Creek group ; whether this group as shown here 

 is only a portion of the great Lignitic group or not, the few observa- 

 tions we have made do not determine. The i)lants found in a number of 

 localities in Monument Creek Valley and near Colorado Springs indi- 

 cate that it is a part of that group — perhaps the upper portion. This 

 group contains beds and seams of impure coal, with deciduous leaves, 

 some of which are identical with species occurring in the Lignitic strata 

 from New Mexico to the Upper Missouri. Indeed, the general aspect of 

 the rocks in this region is much like the Lignitic group on the Yellow- 

 stone and Missouri Elvers near their junction and in the vicinity. The 

 Monument Creek group has not yet yielded many fossils, and these are 

 not usually well preserved. The rocks are extremely soft, usually dis- 

 integrating ea^sily, and too coarse in texture to x)reserve plants well. Not 

 an invertebrate fossil has been found as yet, though it is supposed that 

 some very interesting vertebrate remains came from it. More careful 

 explorations will undoubtedly reveal the existence of fossil evidence. 

 Plants are abundant in several localities, and other forms will most prob- 

 ably be found farther east, toward the interior of the basin. 



Unless these deposits are of modern Tertiary age I am at a loss to 

 account for their i)osition in relation to the mietamorphic foot-hills, as 

 well as the older sedimentary beds. A great portion of the Lignitic 

 group has been lifted up at a moderately high angle, as is shown on 

 either side of the divide, but for about ten to twenty miles a group of 

 beds of very coarse texture, an aggregate of crystals of quartz and feld- 

 spar, jut up against the sides of the mountain at an angle of nearly 15"^, 

 but usually not more than 8° or 10°. The beds which lap immediately 

 on the metamorphic rocks have the appearance of coarse feldspar 

 granites, and as we recede eastward from the base of the mountains 

 3 G s 



