188 EEPOET UNITED STATES GEOLOGICAL SURVEY. 



Fox Hills and Fort Pierre Groups in tlie Upper Missouri Eiver rejjion. 

 Four species are new and therefore irrelevant ; iive are common to both 

 the Fort Pierre and Fox Hills Groups ; one ( Cardium speciosum) is known 

 only in the upi>ermost strata of the Fox Hills Group in that region, and, 

 so far as known, only three are there conlined to the Fort Pierre Group 

 alone. 



Of those thirteen species found at Fossil Pidge, only four were dis- 

 covered by myself in any of the higher Fox Hills strata of that district. 

 This fact sliows a very considerable difference between the fauna of the 

 lower and that of the upper portions of the Fox Hills Group as it is de- 

 veloped in Eastern Colorado ; but the other facts cited show that the differ- 

 ence is not sufficient to warrant its separation into two distinct groups, 

 as has been done in the Upper Missouri Eiver region. Furthermore, 

 while certain species seem to be confined to the uppermost strata, both 

 in Colorado and the Upper Missouri Piver region, these strata are so 

 intimately connected with those beneath them, by other fossils, that 

 range from one to the other, as to make any proper separation of them 

 from the Fox Hills Group proper impracticable. In short while the lim- 

 itation of the vertical range of certain species serves as an approximate 

 indication of recognizable horizons within the vertical limits of the Fox 

 Hills Group as developed in Eastern Colorado, the range of other species 

 is such as to connect the whole together into one natural group only. 



Leaving the district between the South Platte and the base of the 

 Pocky Mountains, which I found occupied by the strata of the Fox Hills 

 Group, I crossed that river at Evans and passed down the south side of 

 its valley to the mouth of Bijou Creek, one of its tributaries from the 

 southward. At a point about eighteen miles east of Greeley I found the 

 uppermost strata of the Fox Hills Group in the south valley side of South 

 Platte Piver, and from that point to about six miles farther eastward I 

 continued to see small exposures of the same, most of which were ob- 

 scure. I however recognized about 20 feet in thickness of strata, and 

 the fossils, which were few and imperfect, were quite sufficient to indi- 

 cate an exact equivalency of the strata containing them with those of 

 the upper part of the section at the mouth of the Saint Vrains. Besides 

 these few characteristic invertebrate fossils, I also found fragments of the 

 fucoid Halymenites mcijor in one of the upper layers. The known general 

 dip of the strata of all that region makes it practically certain that the 

 Cretaceous strata pass beneath the level of the streams along a northward 

 and southward line, which may be drawn a couple of miles west of Gree- 

 ley ; that they receive a greater or less thickness of Laramie strata upon 

 them beneath the debris of the plains. Then a gentle rise brings them 

 up again to view in the valley of South Platte Piver, from eighteen to 

 twenty-five miles east of Greeley, as akeady mentioned. They seem then 

 to pass again by a gentle easterly dip beneath the surface of the river, 

 but I did not trace them farther, as my journey led up the valley of 

 Bijou Creek. It is probable, however, that the exposures of these np])er- 

 most of the Fox Hills strata continue at the surface farther down the 

 South Platte, in its immediate valley. Between Greelej" and the point 

 where these Cretaceous strata are exposed, the space is no doubt occu- 

 pied by at least a small portion of the strata of the Laramie Group, 

 which are covered with the debris of the plains, but I found no exposures 

 of Laramie strata until I reached the valley of Bijou Creek, about twelve 

 miles above its mouth. In the higher i)ortions of the valley side of South 

 Platte Eiver, where I hoped in passing to have discovered exposures of 

 Laramie strata, I found many sand dunes or accumulations of apparently 

 wind-drifted sand, such as those that have been akeady mentioned as 



