200 EEPORT UNITED STATES GEOLOGICAL SURVEY. 



precise boundaries of all of thein, from tlie base of the Red Beds to the 

 top of the Monument Creek Group, were found to be obscure, even where 

 they could be best observed. It is true that the opportunities for such ob- 

 servation are limited by prevailing- dSbris, but in no place where opportu- 

 nity has ottered have I been able to select any stratum or any line or plane 

 of demarkation between any two strata, and say with coniidence that it 

 constitutes the precise boundary between any two of these groups or for- 

 mations. Besides this, the horizons which are indicated by the pres- 

 ence, or known vertical limits of range, of certain species of fossils are 

 all parallel with the assumed planes of demarkation between the groups, 

 and consequently with each other. Thus, for example, a well-character- 

 ized fossiliferous horizon occurs near the top of the Fox Hills Group and 

 another near the base of the Laramie Group, both being constant 

 throughout a large part of this region. They are comparatively near 

 together, no distinct plane of demarkation existing between the two, as 

 before stated, and yet no two fossiliferous horizons in any series more 

 plainly belong to different groups. In the present state of our knowledge 

 edge it is perhaps too much to say positively that sedimentation was con- 

 tinuous and uninterrupted over the area that now constitutes the region I 

 have examined, from the beginning of the epoch of the Bed Beds to the close 

 of the Laramie period, and, x>erhaps, also, to the close of the Monument 

 Creek epoch, but such now seems to have been the case. That oscilla- 

 tions of land-level took place within that time, which shifted the eastern, 

 and doubtless other shore-hues of then existing seas, is certain, as will 

 be hereafter shown, but such oscillations do not seem to have interrupted 

 or materially affected the continuity of sedimentation in the area that now 

 constitutes the portion of Colorado which lies east of the Bocky Mountains. 

 Important physical changes of course took place elsewhere, which had 

 their effect in producing the changes of the faunas of the passing epochs 

 which are represented by the groups in question, and in defining those 

 epochs themselves, but that subject will be briefly discussed on following 

 pages. The observations which I made there indicate that all 'the move- 

 ments which resulted in the elevation of the Bocky Mountain range 

 certainly took place after the close of the Laramie period, and at least in 

 large part after the close of the epoch of the Monument Creek Group.* 

 The faults and trap outflows, however, that have just been noticed, took 

 place at a much later epoch, probably as late as the Fliocene Tertiary, 

 and were probably contemporaneous with a large part of those that are 

 found on the west side of the Bocky Mountains. 



Before taking leave of the east side it is proper to refer to certain drift 

 phenomena which I observed along the eastern base of the mountains, 

 because I shall also have occasion to refer to this subject briefly when 

 treating of regions farther westward. 



Near the base of the mountains, and sometimes reaching several miles 

 out upon the plains, are beds of drift, composed of gravel, usually coarse, 

 and small bowlders. Almost without exception, this material is com- 



* As to whether the Monument Creek Group is really conformable upon the Laramie, 

 I can only say that it appeared to be so where I examined it. It is probably a consid- 

 erably later deposit than the Laramie. 



There is, as reported by the late Mr. Marvinc, a limited unconformity of the Laramie 

 strata upon those of the Fox Hills Group in Middle Park ; but that is believed to have 

 been caused by comparatively slight movements that took place previous to those of 

 the Eocky Mountain uplift proper. Very extensive displacements, however, are 

 known to have taken place at or near the close of the Laramie period in the region 

 that now constitutes the western portion of Green Eiver Basin, as will be shown on 

 following pages ; but even those movements are believed not to have interrupted con- 

 tinuous sedimentation in comparatively large areas. 



