270 FieLp CoLtumpian Mustum—GeEotocy, Vot. I. 
pronounced, and the clays take on a more sandy nature. Just below 
the capping ledge of sandstone there is often a thin stratum of darker 
shale which bears traces of carbonaceous matter; at other places it 
takes on a mottled color and a pebbly texture. 
The Jurassic clays are capped by a massive ledge of sandstone 
varying in this locality from ten to twenty feet in thickness, It is” 
coarse-grained in texture and abounds in quartzitic nodules often so 
massive as to occupy the greater part of the ledge. When freshly 
broken it has a decided yellowish color, but after weathering it changes 
to a dark brown. At the base there is often a stratum of fine-grained 
and more homogeneous sandstone. In certain localities this ledge 
contains the wealth of fossil leaves which everywhere distinguishes 
the Dakota sandstone. 
With regard to the line of demarkation between the Jurassic and 
Cretaceous, the writer cannot agree with the determination of the 
Hayden Survey in this locality. Writing of the Jurassic in the upper 
valley of the Gunnison, Peale says:* ‘‘The massive yellow silicious 
sandstone, in some places quartzitic, at the base of the Cretaceous, is 
so well defined lithologically that there has never been any difficulty 
in separating it from the overlying shales. Along the edge of the 
plains in Colorado it is underlaid by greenish, shaly beds, sometimes 
lignitic toward the top, generally in part or wholly covered, which 
have always been referred to the upper part of the Jurassic.” This 
description will apply to the same formation in the Grand River 
Valley, but writing of the Grand River Valley a year later the same 
writer says: ‘‘The line separating the Jurassic from the overly- 
ing Dakota is indefinite.”+ The following section referred to the 
Cretaceous shows how far he has digressed from the conclusions of 
the previous year :— 
“The following is the average of the Dakota group in the hog-backs 
of the Grand River: 
1. Massive yellow silicious sandstone with faint impressions of 
dicotyledonous stems at some localities - - 3 - 
90-200 
2. Variegated marls with bands of reddish sandstone. These 
beds have the characteristic weathering of bad lands. - 
3. Dark colored sandstones with bands of marls and shales. 
Near the base a layer containing nodules in which there f 50-100 
are Jaspers - - - - < Soe - ‘ s : 
Total thickness - - - - - - - 140-300” 


*U.S. Geological and Geographical Survey of Colorado and the Adjacent Territory, 1875, p- 88. 
yU. S. Geological and Geographical Survey, 1876, p. 180. 
