HATDEN.] GEOLOGY FOX HILLS GROUP. 35 



In the lower portion of Cretaceous No. 5, or the Fox Hills group, the 

 sediments all show a moderately deep sea and quiet waters, in which 

 the various forms of Mollusca peculiar to this group flourished in great 

 abundance, and have been preserved with wonderful perfection. But as 

 we pass upward, we begin to observe signs of a gradual change to shal- 

 low and even turbulent waters. Tracks and trails of worms, etc., are 

 seen on the surface of the thin layers of sandstone, and the more mass- 

 ive sandstones become concretionary, irregular, sometimes quite thick, 

 and then suddenly thinning out so as to be unimportant or entirel^^ 

 absent. While many of the species peculiar to No. 4 as well as No. 5 

 continue to flourish to a certain extent, new forms are introduced, such 

 as Tancredia americana, Gardium specioswn, Mactra formosa, Mactra altaj 

 and many others, previously known to occur in no other locality in this 

 country except near the mouth of the Judith Eiver on the Upper Mis- 

 souri. There are also mingled with them BactiUtes, Ammonites, Inoce- 

 ramus, etc., forms well known in the Fox Hills group all over the West. 

 We may continue our way northward to Cheyenne, and from thence to 

 the Missouri and the Yellowstone region to the north line of the United 

 States, and we shall find the Lignitic group remaining substantially the 

 same and bearing similar relations to the Fox Hills group below. In 

 all this distance, the only break in the connection that occurs is a dis- 

 tance of about two hundred miles, between Cheyenne and the North 

 Platte, where the Lignitic group is overlapped by the more modern beds 

 of the White Eiver group. 



Our investigations in Colorado seem to warrant the following conclu- 

 sions : — 



1st. That through the upper portion of the Fox Hills group, there are 

 clear proofs of a radical physical change, though very gradual, usually 

 with no break in the sequence of time. In this portion of the group are 

 well-marked Cretaceous fossils of purely marine types, and no others. 



2d. That above the upper Fox Hills group, there are about 200 feet of 

 barren beds, which may be regarded as beds of passage to the Lignitic 

 group, which more properly belong with the Fox Hills group below. In 

 this group of transition beds, all trace of the abundant invertebrate life 

 of the great Cretaceous series below has disappeared. 



3d. In almost all cases we find at the base of the true Lignitic group 

 a bed of sandstone, very irregular in thickness and structure, which 

 seems to mark the horizon or dawn of this group. In this sandstone, 

 the first deciduous leaves peculiar to this group occur. No purely ma- 

 rine Mollusca pass above this horizon. Estuary or brackish-water shells 

 are found in many localities in great abundance. These soon disappear, 

 and are siicceeded farther north by fossils of iDurely fresh-water origin. 



Whatever view we may take in regard to the age of the Lignitic group, 

 we may certainly claim that it forms one of the time-boundaries in the 

 geological history of our western continent. It may matter little whether 

 we call it Upper Cretaceous or Lower Eocene, so far as the final result 

 is concerned. We know that it plays an important, and, to a certain extent,, 

 an independent part in the physical history of the growth of the conti- 

 nent. Even the vertebrate-paleontologists, who pronounce with great 

 Ijositiveness the Cretaceous age of the Lignitic group, do not claim that 

 a single species of vertebrate animal passes above the horizon I have 

 defined from the well-marked Cretaceous group below. 



Having presented these facts as briefly and clearly as we were able, 

 we will leave the further discussion of the age of the group to a future 

 period. 



