128 GEOLOGICAL SURVEY OF THE TERRITORIES. 



resembling this, and on lithological grounds would be appropriately 

 grouped with it. In fact they have been considered Jurassic, and the 

 only Jurassic rocks in this region, by the geologist who claims to have 

 discovered the representatives of this formation in JSTew Mexico. Unfor- 

 tunately, however, for that classification, immediately over the thin 

 stratum of yellow sandstone which overlies the coal are beds of clay- 

 shale with bands of limestonS in which are unmistakable Cretaceous 

 fossils. It is, therefore, evident that the Jurassic formation cannot be 

 extended in this direction, and there is no alternative left but to con- 

 sider the coal-seam, if Jurassic, the sole representative of the Jurassic 

 series, or to combine with it some portion of the underlying variegated 

 marls, which, for this purpose, must be abstracted from the Trias of 

 Mr. Marcou." 



CRETACEOUS. 



It is, perhaps, impossible at present to subdivide the Cretaceous 

 formation, as seen west of the continental divide, in the same manner 

 as has been done east of the mountains. 



For the sake of convenience in description, it may be best to consider 

 it as divided into three groups, Lower, Middle, and Upper Cretaceous, 

 as follows : 



Thickness 

 in feet. 



Lower Cretaceous — Dakota group (No. 1) 500-700 



( Fort Benton group (No. 2), ) 



Middle Cretaceous — < Nioorara division (No. 3), > 2,000 



( Fort Pierre group (No. 4), ) 



fFox Hillbeds (No. 5). ~^ 



TT,..,^.. n..„4-„^„^„„ A series of slialy sandstones, which in the lower ! ^ c-nn a nnn 

 Upper Cretaceous — < . v v.- /-> a iu -j. /■> ^ ^.i • > ...1,500-2, 000 



^^ 1 part are lignitic. On Anthracite Creek this ( 



[^ lignite is changed into anthracite coal J 



Total 4,000-4,700 



This table represents the estimated thicknesses as developed in our 

 district. 



LOWER CRETACEOUS. 



Dakota group — Formation No. 1. 



■ Immediately above the group of shales last described, under the head 

 of Jurassic and conformable to it, is a series of beds in which rather mas- 

 sive siliceous sandstones predominate. It is persistent throughout the 

 Eocky Mountains, preserving its lithological characters very constantly 

 over widely-separated areas. 



The group forms a convenient horizon for reference, being more 

 strongly marked, perhaps, than any other in the sedimentary series. Its 

 age is well established, and I will therefore not take the space here to 

 repeat the evidence. The discussion in full will be found in Professor 

 Lesquereux's " Cretaceous Flora of the West." * 



The evidence as to its identity in Colorado is as follows : 



During the explorations of 1873, near the exit of the South Platte 

 Eiver from the mountains, I found fragments of a Proteoides,i of which 

 Professor Lesquereux, writing me, said : " It is very near Froteoides 

 acuta (Heer.), if not a small form of the same." 



As yet no leaf of this genus has been found higher than the 



* Report of United States Geological Survey, vol, vi; Cretaceous Flora, by Leo 

 Lesquereux. 

 t See Report of United States Geological Survey for 1873, page 196. 



