216 GEOLOGICAL SURVEY OF THE TERRITORIES. 



was formed not very long after the deposition of the strata composing 

 that formation. As to the causes to which it owes its existence, 1 have 

 no explanation to offer, although I think it highly probable that its 

 occurrence is in intimate connection with the production of the meta- 

 morphic rocks, the formation of which certainly fell into some period 

 subsequent to the Devonian epoch. 



Overlying the last-named formation, on the east side of the Animas, 

 we find the Carboniferous beds. Analogous to those on the west side, 

 if not identical, they commence with a series of sandstones and shales 

 containing fragments of plants. Isolated interstrata of fossiliferous 

 limestones occur. Higher up the heavier limestone strata are reached, 

 and then follows the red sandstone. Referring to section I, given under 

 the head of Devonian, we see that there is a white, coarse-grained, 

 quartzitic sandstone, #, deposited upon the Upper Devonian limestone. 

 This 1 regard as a good horizon to mark the beginning of the Carbonif- 

 erous. Above it follow yellow to brown sandy shales, h, weathering in 

 small tabular fragments, attaining higher up more of the sandstone 

 character, i <?., losing that of shales. Thin beds of limestone, coutainiug 

 corals, occur in the sandstones. Upon the next highest stratum, i, sta- 

 tion 49 was located, at an elevation of 11,700 feet. It is a dark-blue 

 limestone, containing Athyris subtilida. Its thickness is not great, 

 amounting to 80 to 100 feet. It is covered by yellow sandstone, /, in 

 which numerous fragments of plants, probably belonging to JEquisetum, 

 were observed. Yellow to brown sandy shales, m, and marls follow 

 above this, underlying the red sandstone, n. Of this but little can 

 be said, inasmuch as its stratigraphical conditions are very uniform, 

 and the variations shown by its single beds but slight. Some of 

 the latter show more of a shaly character, in that case having a 

 darker color; others are more coarse-grained than is usually the 

 case. The thickness of all these beds will amount to over 3,000 

 feet, of which nearly 2,000 belong to the red sandstone. Station 40 

 is located on this sandstone, toward the lower end of Auimas Park, 

 on the east side of the river. Dipping southward at an angle of 

 about 6°, it does not quite reach down to the valley, but permits 

 the underlying limestones and shales to crop out. Farther east they 

 seem to pinch out and are not cut by section I. Descending from that 

 station Athyris subtilida was found in positu in a thin, shaly stratum 

 within the red sandstones, nearly one thousand feet above its 

 lower limits. A short distance below the latter, blue limestone set in, 

 containing numerous specimens of Productus semistriatus, Athyris sub- 

 tilida, and Spirifer. One Productus semistriatus was found on station 

 40. i. e., in the highest third of the red sandstones. It was not in posi- 

 tion, however, and although I cannot conceive of any plausible method 

 by which it might have been transported thence, I hesitate to accept 

 it as positive evidence. A short distance south of station 40 the sand- 

 stone reaches the valley, and the underlying beds are hidden from view. 

 The thickness of the Carboniferous strata exposed at station 40 is about 

 eighteen hundred to nineteen hundred feet, nearly fourteen hundred feet 

 of which are * formed by the red sandstone. The eastern limit of the 

 Carboniferous occurs near the Rio Vallecito and over toward Rio Pinos. 

 Owing to very dense timber, fewer observations could be made than 

 might have been desirable, but enough was seen to show that both ver- 

 tical development and stratigraphy remain unaltered. So far as could 

 be determined, none of the Carboniferous strata were altered by the ex- 

 tensive metamorphosis that took place farther north. 



Ascending from Baker's Park the pass that leads into the lower val- 



