Mesozoic and Ccenozoic Geology and Palceontology. - 101 



there is a large thickness of the strata filled with concretions that 

 tire made up mosth" of an aggregate of fossils, as Ammonites, Bacii- 

 lites, etc. Near Chain de Roche creek these concretions have been 

 swept down into the Missouri b}^ the swift current, during the spring 

 floods, and in the low water of autumn the}' present a picturesque 

 appearance. 



Although the rivers cut deep channels through the different forma- 

 tions, we do not meet with the Fox Hills Group along the Missouri, 

 until we reach nearly up to the mouth of Cannon Ball river, 3'et fifty 

 miles or more before reaching that point it has overlapped the Fort 

 Pierre Group. In traveling across the plain countr^^ westward from 

 Fort Pierre, we find it occupying the entire area. Very soon after pass- 

 ing west of the Big Cheyenne river the traveler will readily recognize 

 Its presence by the more cheerful appearance that it gives to the sur- 

 face, as well as b}^ the greatly increased growth of vegetation. The 

 water is pure and good, and springs become quite common in the hills. 



An important feature in the geology of the West is the great lake 

 basins, which seem to set in the older formations and in each other 

 like dishes. The principal one is the Fort Union, or Great Lignitic 

 Group, which forms the transition group, from the strictly marine con- 

 dition of the Cretaceous period, to the epoch of the numerous fresh- 

 water lakes, which were scattered all over the country west of the Mis- 

 sissippi. This group was called the Fort Union or Lignitic Group in 

 1861,* and supposed to be of Eocene age. 



It was" described as consisting of beds of clay and sand, with round 

 ferruginous concretions, and numerous beds, seams and local deposits 

 of lignite, great numbers of dicotyledonous leaves, stems, etc., of the 

 genera Platanus, Acei\ Ulmiis, Populus, etc., with very large leaves 

 ■of true fan palms; also Helix, Melania, Vivipara, Corbicula, Unio, 

 Ostrea, Cor6z(Za and scales o^ Lejyidotus, ^ith bones of Trio)iyx, Emys^ 

 Compsemys, Crocodilus, etc; as occupying the whole country around 

 Fort Union, extending north into the British possessions to unknown 

 distances, southward to Fort Clark, under the White River Group on 

 North Platte river above Fort Laramie, and on the west side of Wind 

 River mountains; and as having a thickness of 2,000 feet or more. The 

 passag-e from the brackish to the fresh water beds seems not to be 

 marked by any material alteration, in the nature of the sediments; nor 

 have we any reason for believing, that any climatic or other important 

 physical changes bej^ond the slow rising of the land, and the conse- 



Proc, Acad. Xat, Sci, 



