126 GEOLOGICAL SUEVEY OF THE TEREITOEIES. 



Just above tlie bed marked ISTo. 10 in the section is a massive sand- 

 stone, from 60 to 70 feet thick, which forms the summit of a prominent 

 hog-back, beyond which the beds are concealed. I hnve taken this bed 

 as the lower portion of the Dakota group. It is possible that on further 

 investigation the layers marked 8, 9, and 10 may have to be included 

 in the l)akota group. This would give a total thickness for the Juras- 

 sic at this point 280 feet instead of 440 feet. 



Below the first layer in the section is a coarse pink sandstone, which 

 at soQie points is a conglomerate. It rests immediately on the red 

 sandstones, and I have taken it as the top of the Triassic. Of coarse, 

 without the evidence of fossils, the lines separating the Jurassic from 

 the Cretaceous and the Triassic must be necessarily, somewhat indefi- 

 nite. The lines I have taken are therefore arbitrary and liable to be 

 changed when more complete data are obtained 



In the section given above, and in that .made on Eagle River, it will 

 be noticed that the arenaceous element seems to predominate. The 

 beds were so much concealed that it was impossible to make a more 

 detailed section. The extension of the hog-backs up Rock Creek into 

 the Elk Mountains will be treated of in the reports of Dr. Havden and 

 Mr. W. H. Holmes. 



Gunnisooi River. — The Jurassic appears on the Gunnison first in the 

 Grand Canon, resting immediately on the schists, and gradually becom- 

 ing thicker as we go down the river, until the Red Beds apjjear between 

 it and the schists. The Jurassic is also exposed on Smith's Fork, 

 extending some distance from the mouth up the stream. 



The area occupied by the formation, in counectiou with the Red Beds, 

 is shown on map B. 



I was unable to make any section in the course of the canon, but, as 

 seen from station No. 80, it appears to consist of variegated yellow, 

 white, pink, and gray beds, i)rol)ably sandstones, shales, and marls. On 

 the eastern side of the canon these beds have a much greater extension 

 than on the west. The entire thickness is probably about the same as 

 in the lower caQon, as shown in the section to be given farther on. 



On Saiith's Fork, the variegated appearance of the Jurassic strata is 

 also seen. In the sections made on Eagle River and Roaring Fork, this 

 is wanting. I shall refer to this fact again. In the lower caiion the 

 Gunnison very soon cuts through the Dakota group, which, at tire head 

 of the caiion, forms the bluffs on either side, and reaches the soft 

 Jurassic layers beneath. 



At first they are exposed in isolated patches similar to the outcrops 

 of the Red Beds lower down, varying according to the curves of the 

 river. When the Red Beds are reached, however, the Jurassic is shown 

 on both sides of the stream. 



No. 9. — Section of Jurassic in llie loicer canon of Gunnison River near 



Station (>0. 



Base. Thickness. 



Ft. In.. 



1. Soft greeni>sh and purplish argillaceons sandstone.s aboxit 20 



2. Space lilled with gray laminated limestones and interlaminated soft gyp- 



siferous shales 80 to 90 



3. Compact white siliceous sandstone 8 9- 



4. Soft argillaceous and arenaceous shales, with bands of hard sandstone 



from 6 inches to a ff)ot in thicknes.'S. The shales are covered with an 

 efflorescence of alkali, in which there is salt, as revealed in tastingit.. . 30 



5. Compact white siliceous sandstone, like that marked No. 3 6 10 



6. Dull bluish-gray limestone, in layers of about a foot thickness, having 



shaly arenaceous and argillaceous beds between. Near the top are soft 



greenish and pink shales. All the beds are more or less gypsiferous.. 35 



