94 CARBONIFKROUS FORMATIONS AND FAUNAS OF COLORADO. 



thickness of but 2,500 feet and does not mention interbedded limestones. In both 

 particulars this agrees especially with the Weber grits formation alone. Still 

 farther south, on the other hand, fossiliferous limcststonos arc present, as we have 

 just seen. Trustworthj^ and detailed information about the whole range south of 

 the Leadville region is, unfortunatel}', sadly lacking, but I judge that the Upper 

 Carboniferous sandstones and conglomerates, represented southward by the Arkan- 

 sas sandstone, are thinner in the southern part, and suspect that this will prove to 

 be due to erosion of the upper surface by one or several combined of the erosion 

 periods that appear to have succeeded the deposition of the Maroon sediments. 

 Stevenson found no Triassic in this region, and, in fact, the bright Ked Beds appear 

 to be in large measure wanting. Thej^ are, however, foUnd in the Tenmile region, 

 and their apparent absence farther south is possibly owing to the same cause by 

 which the thickness of the Maroon sediments appears to have been seriously dimin- 

 ished.'* Wherever any representation of the Wj'oming formation occurs in the ax'ea 

 examined by Stevenson it was evidently included by him in the Carboniferous. 



In the supplement to volume 3 of the reports of the Wheeler survey' Steven- 

 son described the geologj'^ of an extensive territory in New Mexico and southern 

 Colorado. It probably included in the latter State all the mountainous portion 

 south of north latitude 37° 20'. Two Carboniferous areas are described, one of 

 which is situated entirely in Colorado and the other entirely in New Mexico. The 

 Colorado area, which is described in less detail than the other, occurs on the east- 

 ern slopes of the chain of mountains called in the Haj^'den atlas the Sangre de 

 Cristo Range, and extends from Spanish Peaks to Costillo Peak. The Carbonifer- 

 ous beds rest upon the Archean. . The base of the series is a conglomerate sandstone 

 not far from 800 feet thick which contains some beds of fine-grained red sand- 

 stone and red shale. Above this comes in the lowest limestone, which is 7 feet 

 thick, blue, slatj^, hard, and somewhat fossiliferous. Next occurs an interval of 

 700 feet filled with red shale and sandstone. This is succeeded by a blue, very 

 compact limestone, of which 20 feet are exposed, showing some calcspar but no 

 fossils. Above the limestone occur 300 feet of red sandstone and reddish shale. 

 Next follows another limestone which is gray, not verj- hard, richly fossiliferous, 

 and not far from 100 feet thick. Above the limestone are about 1,800 feet of 

 reddish, thin-bedded, fine-grained sandstones, holding beds of red shales. The 

 rocks thus far mentioned, which have a total thickness of 3,727 feet, seem to 



rtl have suggested the probability of an extensive time break and period of erosion between the brick-red or 

 "Triassic" Red Beds and the Maroon formation. To this the reduction in the thickness of the upper JIaroon from 2,500 

 feet in the Crested Butte quadrangle to 300 feet in the San Juan region (Rico formation ) has been ascribed. Possibly the 

 same influence was felt to the east and the Maroon reduced before the deposition of the "Triassic," which has been 

 itself removed. The great thickness of the Maroon (Arkansas sandstone) farther south in the Sangre de Cristo would 

 rather seem to indicate that the thinness of these beds in the region of the Arkansas was a local matter and probably the 

 result of comparatively recent causes. 



b V. S. Geog. Geol. Surv. W. 100th Mer., Rept., vol. 3, Suppl.. 18S1. 



