20 GEOLOGICAL SURVEY OF, THE TERRITORIES. 



quality of its fuel, simply to distinguish it from the other great group 

 of older date, the age of which is not questioned. 



The time has now come, as it seems to me, when the materials are so 

 abundant that the subject can be reviewed with some care. It is well 

 known that I have held with some tenacity the opinion that the coal- 

 formations of the West are of Tertiary age ; and I still regard the Lig- 

 nitic group proper as transitional or Lower Eocene, and shall so regard 

 its age until the evidence to the contrary is much strouger than any 

 which has been presented up to the present time. When, however, the 

 proof is sufficient to decide the Cretaceous age of the group, I shall 

 accept the verdict without hesitation. It is somewhat doubtful whether 

 the age will ever be decided positively to the satisfaction of all parties; 

 still we shall see in the course of this report that the character of the 

 paleontological as well as the strictly geological evidence is such that it 

 is not a matter of importance whether the entire group be placed in the 

 Lower Tertiary or Upper Cretaceous, and it is most probable that the 

 testimony of the different paleontologists will always be as conflicting 

 as it is at present. 



In order that the reasons for my belief in the Tertiary age of the 

 Lignitic group may be more clearly understood and harmonized with 

 the present state of our knowledge of the subject, I will give a brief 

 history of the commencement and progress of its examination. 



My first knowledge of this group was obtained in the summer of 1854, 

 when I made a somewhat careful examination of the beds from their 

 first appearance on the Missouri River near Fort Clarke to the mouth of 

 the Yellowstone, and thence up that river to a point near the mouth of the 

 Big Horn. In all this distance, about six hundred miles, following the 

 windings of the river, the Cretaceous beds appear but once, and then only 

 along the bed of the river for a few miles, while the entire country, with 

 this exception, is occupied with the Lignitic group. The area of this forma- 

 tion on the Upper Missouri cannot be less than one hundred thousand 

 square miles, and extends far north across the northern boundary of the 

 United States into the British possessions. This group everywhere rests 

 upon well-defined Cretaceous beds, which we have ail along regarded as the 

 highest known in the West, and have received the name of the Fox 

 Hills group, from a locality on the Missouri Eiver called the Fox Hills, 

 or Fox Ridge, where this formation was first studied, and was very lull 

 of molluscan life. There is a gradual passage upward from the black 

 plastic, shaly clays of No. 4, or the Fort Pierre group, to the yellow cal- 

 careous clays of the Fox Hills group, and at the upper portion, the 

 sediments are arranged in thin layers, very arenaceous, and indicative 

 of their deposition in turbulent as well as shallow waters. In these are- 

 naceous sediments, the well-marked marine life ceases to exist, and soon 

 after appear the brackish-water species. Between the Big Cheyenne 

 and the Moreau Rivers, branches of the Missouri that come in from the 

 west side, the Lignitic Strata overlap those of Cretaceous age, and in the 

 lower beds occurs a species of Ostrea associated with some other brackish- 

 water forms. I am not positive as to the exact position of these fossils, 

 but I am confident that a bed of gray sandstone, with a layer of impure 

 coal or Lignite lie, below any of the brackish-water forms found in the 

 Northwest. Scattered over the weathered surface of these Lower Lig- 

 nitic beds, and believed, without doubt, to have been originally 

 imbedded in them, were found several specimens of Vertebrata which 

 have been regarded by Protessor Cope as characteristic of the Creta- 

 ceous era. So far as the Northwest is concerned, the brackish-water 

 beds are not more than 200 feet in thickness, while those that are 



