Ilesozoic and Cmnozoic Geology and Palaeontology . 177 



perpendicular, and which ramify in all directions. The banks of Milk 

 river rise abruptly nearly 300 feet above the level of the stream, and 

 are more than a mile apart. Sections of the Fort Union Group were 

 obtained near the 49th parallel 284 feet in thickness. In the coulees 

 and gorges which intersect the prairie on the west side of the Milk 

 river, exposures of the same group continue to occur for many miles. 

 Near the 620 mile point, west of Red river, a very interesting and 

 highly fossiliferous section of the brackish water deposits of the Fort 

 Union Group is exposed. In the valleys which seam the flanks of 

 the hills, and furrow the surface of the prairie around East Butte 

 numerous more or less extensive exposures of this group occur. But 

 on the west side of West Butte, where a considerable bixjok issues from 

 the central valley, a section of the Fort Pierre Group is exposed, 800 

 feet in thickness. 



The exposures of the Fort Union Group continue to occur as we go 

 west until the base of the Rocky Mountains is reached. - The}^ occur 

 on the branches of Milk river, St. Mary river and the Belly river. 



Prof. G. M. Dawson found the Lignitic or Fort Union Group every- 

 where conformable with the Fox Hills Group below. He referred it to 

 Tertiar}- age, and estimated the thickness, assuming the horizontality 

 of the beds and the rise in the general surface of the 'countrj^, at not less 

 than 1,000 feet. 



Dr. J. W. Dawson* described, from the Fort Union Group, south of 

 Woody Mountain,' Lemna sczitata, and from west of Woody Mountain, 

 .j^sculus antiqua. 



Prof. E. D. Copef described, from the Fort Union Group, six 

 miles west of First Branch of Milk river, near latitude 49°, Gion- 

 odon stenopsis^ Compsemy^ ogmius, and from the Bad Lands of South 

 Wood}^ Mountain, Plastomenus coalescens, and P. costatus. 



Speaking of the age of the Fort Union or Lignitic Group, the 

 Bitter creek series and the Bear River Group he ^^y^X that Prof. 

 Lesquereux, as is well known, pronounced this whole series of forma- 

 tions to be of Tertiary age. The material (fossil plants) on which 

 this determination is based is abundant, and it must be accepted as 

 demonstrated beyond all doubt. But that he regarded the evidence 

 derived from the mollusks in the lower beds and the vertebrates in 

 the higher as equall}^ conclusive that the beds are of Cretaceous 



=•■■ Rep. Geo., 49th parallel. 



t Geo. Rep., 4r9th parallel. 



X Vert. Cret. Form, of the West.— Hayden's U. S. Geo. Sur. Terr., vol. 2. 



