174 REPORT UNITED STATES GEOLOGICAL SURVEY. 



the valley for a distance of five or six miles, and which constitute the 

 Crow Creek section, is frequently cut from top to base by gullies leading 

 directly across the exposures into the creek, and bed No. 5 is nowhere else 

 recognized, although the beds which immediately underlie and overlie it 

 respectively are at those other points clearly distinguishable and in con- 

 tact. The prevalence of one and the same species of Ostrea, both above 

 and beneath the fresh- water bed, also suggests the limited extent of that 

 fresh-water deposit and the unbroken continuity of sedimentary deposi- 

 sition in at least partially saline waters at no great distance away from 

 the limited area in which the fresh-water deposit was made. Although 

 the border of this bed is thus recognized at the Crow Creek locality, it 

 does not seem to possess the characteristics of a true littoral deposit, 

 such as water-worn pebbles, marks of wave action, &c. The presence 

 in it of the insect egg-masses, palustral shells, and fragments of wood 

 and deciduous leaves, however, seem to indicate that a shore-line was not 

 far distant. 



A marked peculiarity of the fauna of the Crow Creek locality is 

 the great prevalence of Corhicula, including both the typical forms of 

 the genus and those of the subgenus Leptesflies. The recognized species 

 are six in all, and some of them appear to be intermediate in character 

 between the typical forms and those of the subgenus Leptesthes, and one 

 of them approaches the form to which Mr. Meek has given the subgeneric 

 designation of Yeloritina. 



Other important observations were made in relation to the Laramie 

 Group and its fossils in the valley of Bijou Creek, and elsewhere east of 

 the Eocky Mountains, but the plan adopted for my report is to make the 

 record of my observations in the order of my line of travel. Therefore 

 the Cretaceous rocks of the valley of the Cache a la Poudre and other 

 localities will now be discussed, and the consideration of the Laramie 

 Group and its fossils will be resumed on following pages. 



Eeturning from the valley of Crow Creek to the Cache a la Poudre, I 

 passed up the south side of its valley, by way of Greeley, to a point 

 about five miles westward from the town. Here, and also at intervals 

 within a distance of six or seven miles farther westward, I found the 

 upper series of Cretaceous strata exposed. From these exposures I 

 collected quite a number of species of invertebrate fossils, some of which 

 respectively characterize the different divisions of the Cretaceous series 

 that have been recognized in the Upper Missouri Eiver region, from and 

 iucluding the Fort Pierre Group upward. Those obtained in the valley 

 of the Cache a, la Poudre within the limits just named were collected 

 mainly at, and in the vicinity of, the farms of Frank Marcks and Aaron 

 Eaton, respectively. 



The dip of the strata in this region is gently to the eastward, but as 

 it is a little greater than the coincident slope of the stream, one comes 

 upon lower and lower strata as he passes westward up the valley. The 

 broad lower lands of the valley are covered with alluvium, and the 

 higher surfaces with the usual prevailing debris of the plains, so that 

 the exposures of strata, even in the valley side, are very few, and the 

 thickness exposed at any one locality is very small. Much the most 

 important exposure, as regards extent and thickness of strata, that I 

 observed in this valley, was found at and in the vicinity of Mr. Eaton's 

 farm, where it forms a precipitous bluff near the river, and shows a 

 thickness of strata amounting to about 100 feet. The fossiliferous layers 

 of this exposure are few and limited, but traces, at least, of fossils occur 

 throughout the whole thickness. No continuous measurements of the 

 strata exposed in the valley side within the distance named- could be 



