403 



and rusty-brown concretionary sandstones, there is an abundance of 

 impressions of sea-weeds, especially the Ralymenites major of Lesque- 

 reux. The changes in the physical conditions, daring this period of the 

 deposition'of the sediments, are admirably well shown by the different 

 layers of the upper portion of the Fox Hills group up to the lower sand- 

 stones of the Lignitic. 



The next most favorable locality for examination is near Platteville, 

 on the Denver Pacific Eailroad, about forty miles north of Denver. We 

 have studied with much care the entire area embraced in the drainage 

 of the Big and Little Thompson Creeks, Cache-a-la-Poudre, »&c., so that 

 it is hardly possible we can have been mistaken in our conclusions. On 

 the west side of the South Platte Eiver, opposite Fort Saint Vrain, are 

 some rather high bluff hills, composed of the Upper Fox Hills group 

 entirely. Here is an abundance of well-marked Cretaceous fossils, some 

 of which have a wide geographical distribution, and are characteristic 

 of the entire Fox Hills group, but which as a whole are peculiar and 

 have not been known to occur previously in any other locality, except 

 at the mouth of the Judith, on the Upper Missouri River, The entire 

 mass of rocks in this region and on the Cache-a-la-Poudre show plainly 

 the gradual approach from sediments deposited in deep, quiet waters to 

 those laid down in shallow and turbulent seas. The shells in the Upper 

 Fox Hills group were for the most part carried into eddies or restricted 

 depressions, where they are found now, much broken and worn. There 

 is no particular horizon in the group where the fossils may be found ; 

 but here and there a layer, from an inch to a foot in thickness and only a 

 few feet in horizontal extent, is almost entirely made up of the shells. 

 Of the fossils found together in the same beds N'ucula ccmcellata, N. 

 planimarginata,A[actra Warrenana, M. fonnosa, Cardium speciosum, Tan- 

 credia Americana are most abundant. A species of Ammonite, J., lohatus, 

 which has a very extended vertical range in this group, is one of the last 

 to disappear. It is found at this locality near the summit of the group. 

 About five miles to the west of this point, on the opposte or east side of 

 the South Platte, several shafts have been sunk in search of coal. The 

 interval is mostly the alluvial bottom of the river, and therefore all the 

 basis-rocks are concealed, but in this interval there were originally, with- 

 out doubt, from one to two hundred feet of irregular layers of sand, 

 sandstone, &c., probably barren of fossils. At any rate, the bed of sand- 

 stone, usually called the fucoidal sandstone, which usually lies at the basis 

 of the Lignitic group, is wanting, or is obscured. The bed of coal, 2^ 

 feet thick, was reached 52feet below the surface. ]S'o fossils were found ; 

 but 300 yards southeast of the coal-shafc, the same parties sunk a second 

 shaft, with the following results : 



Ft. In. 



1 . Yellow sand and clay 42 



2. Indurated clay or soapstone, with three inches of charred 



material, called " smut " by the miners 2 



3. Clay .' 4 



4. A layer of shells, Anomia^ Corbicula, «&c 3 6 



S.Clay „ G 



6. Brown sand 1 6 



7. Yellow and gray sandstone 10 . . 



8. Clay or soapstone 2 ' . . 



9. Coal 2 6 



10. Clay-shale 4 



11. Fine gray arenaceous clay 2 



12. Clay near coal 



