220 GECLOGICAL SURVEY OF THE TERRITORIES. 



stoues, extendiDg from stations 36, 37, and 31 southward along the Ari- 

 mosa, its western and northern tributaries, nearly to stations 42 and 43. 

 Denselytimberedridges, with only occasional exposures along steep banks 

 or bluffs, mark the horizontal distribution of the formation under consid- 

 eration. The drainage cuts in deeply, leaving rounded spurs and ridges 

 to direct the future water-courses. Northwest of the junction of the 

 Admosa with the Animas, the first outcrop of the red sandstone near 

 the Animas Valley is observed. There it occurs as the capping of a 

 small hill, separated from the main mass, further south, by the canon of 

 the Arimosa. About four miles south of station 40, the sandstones reach 

 the level of the valley. From the northern limits of the sedimentary 

 formations, down to this point, they have preserved an even dip to the 

 south, varying from 4P to 12°. At this point it amounts to about 5°. 

 As on the east side of the river, the sandstones show interstrata of white 

 beds, arranged in the same vertical succession as there. There can be no 

 doubt as to the identity of the formations on either side of the river. 

 As soon as this point is established, it remains to be determined by 

 what agency they were separated, so as to give rise to the formation of 

 a broad valley between the two exposed walls. It has been mentioned 

 of the Lower Carbonifei'ous, that the bluffs it formed along the valley 

 were nearly vertical. A considerable amount of debris certainly ob- 

 scures the exposure of the face at the base, but presumably the same 

 general direction is continuous. This fact holds good also for the 

 red sandstones on one as well as the other side of the Animas. 

 Inasmuch as the dip and strike of the strata are about the sameon both 

 sides, the former parallel to the general course of the river, and the 

 various beds reach the valley-bottom opposite each other, it is evident 

 that no disturbances took place by which both sides were not equally 

 affected. The agencies by which the valley of the Animas could have l)een 

 formed are: (1) gradual erosion by the river; (2) glacial action of great 

 magnitude; (3) separation by contraction ; (4) separation by directly-act- 

 ing forces. Gradual erosion by the river would certainly be the first to be 

 examinedinto. There wereevidencesfound, on station 40, that at onetime 

 snjall erratic fragments, originating from the quartzitic group, were car- 

 ried alougatan elevatiouof nearly 1,800 feet above the present level of the 

 valley. It is possible, however, that they might have been brought by 

 some of the drainage flowing into the Animas from a northeasterly di- 

 rection. This view is by far more plausible than that they should have 

 been carried there by the Animas itself. On the west side no such frag- 

 ments were observed. In case the valley had been formed by gradual 

 erosion, it would seem natural that those formations yielding most read- 

 ily to decomposing agents, should furnish the broadest portion of it. 

 Such, however, is not the case, neither in the upper nor lower part of 

 the valley. In the chapter on metamorphics, mention has been made 

 of the evidences of former glacial action, and the granitic rocks in the 

 Animas Valley, resembling roches moutonnees, near the head of the park, 

 have been alluded to. I am in doubt, however, whether the latter 

 owe their form to any action but that of gradual decomposition in 

 positu. We have, in this instance, a metamorphic granite, stratified to 

 so considerable an extent that the strata may easily be recognized. In 

 addition to this, large crystals of feldspar are the predominating com- 

 ponent mineral, and the characteristic shape of this granite, now shown 

 at several points, can readily be accounted for, as the result of decom- 

 position progressing under circumstances thus favorable. !N"owhere 

 throughout the region is there any positive evidence that the glacial 

 phenomena were anything but local, and of small extent. Separation 



