130 CARBdNIKEKdTTS FORMATIONS AND FAUNAS OF COLORADO. 



foUowod by the Lower Cretaceous sundstoiie, and also because he in one place 

 iissiyned to the Lower Carboniferous a thickness almost as ^reat as that of Cross and 

 Spencer's Hermosa formation, the inference might be made that his Upper Carbo- 

 niferous represented their Rico, Dolores, and La Plata formations; but his faunal 

 evidence seems to place this out of the question and make it necessary to consider 

 that series part of the Hermosa formation, unless, what is not improbable, his 

 Upper Carboniferous is rex^resented bj^ the Hermosa in one instance and by the 

 Dolores in another. 



The red sandstone is followed by the white sandstone of the Lower Cretaceous, 

 which rests unconformably upon it, and Endlich points out that it was preceded by a 

 deformation of the Paleozoic formations (p. 215). He says: 



"Although the Cretaceous beds of the southern and western portion have the 

 same general direction of dip as the older ones, several localities have furnished evi- 

 dence that the disturbances affecting the Devonian and Carboniferous must have 

 occurred before the deposition of the Cretaceous. It is possible that to this fact, to 

 the higher relative position at the time, the absence of Triassic and Jurassic beds 

 may be attributed" (p. 240). 



In 1875 W. H. Holmes was given charge of the geologic survey of the San Juan 

 division," and his report is found on pages 237 to 276 of the Haj^den Annual Report 

 for that year. The San Juan division is defined as the area which is "bounded on 

 the west by 109° 30', on the south by 36° 4:5', on the north by 37° 50', and on the 

 east by Mr. Wilson's Work of lS7i." Portions of this area lie in the States of Utah, 

 Arizona, and New Mexico, but by far the greater part is in Colorado. 



This territory is chiefly occupied by rocks of post-Paleozoic age, and it is to 

 these, consequently, that most of the discussion is devoted. Carboniferous beds 

 occur, however, overlaid by the Juratrias. To the latter are assigned beds 12, 

 13, 14, and 15 of the general section, with a combined thickness of 1,500 feet. 

 "Group 12 consists of brownish and purplish laminated sandstones, generally less 

 than 200 feet in thickness. This is the fossil-bone bed of Dr. Newberry, which is 

 thought by him to be Triassic. Group 13 is the well-marked bed of massive white 

 and pinkish sandstone. This is succeeded by a laminated series, and this by red 

 sandstones." 



I find it difficult to correlate these subdivisions exactly with the formations 

 recognized by Cross and Spencer in the Rico Mountains, though the general equiva- 

 lence is obvious. It seems a safe hj^pothesis io suppose that the Upper Dakota of 

 Holmes is the same as the Dakota sandstone of Cross and Spencer. His Lower 

 Dakota, therefore, would probably be the same as their McElmo formation. With 

 this, however, the resemblance seems to end. Though the two sets of formations 

 agree singularlj'' in thickness when arranged in parallel columns according to this 



aV. S. Geol. Geog. Surv. Terr., Ninth Ann. Kept., for 1875, 1S77, pp. 237-276. 



