1874.] 247 [Allen. 



these peculiar phenomena of metamorphism much to the northward 

 of the Missouri River. There is, however, an outlying district west 

 of the main chain of the Rocky Mountains, on the Gros Ventres 

 Fork of the Snake River. Throughout this extensive region the 

 strata are nearly always horizontal in position, and consist of soft 

 clays, marls and sands, with occasional beds of soft, friable sand- 

 stone, and thin bands of indurated yellowish clay, interspersed with 

 heavy seams of lignite. Owing to the yielding nature of these mate- 

 rials, the streams have excavated for themselves deep channels, and 

 the country bordering them is deeply cut by innumerable gullies and 

 ravines, extending back often for many miles from the principal water 

 courses, forming the well-known and almost impassable " Bad Lands " 

 of this region. These ravines vary in depth from one hundred to three 

 or four hundred feet, and so extensive has been the denudation at many 

 localities, that only narrow ridges and isolated buttes, with their 

 naked and almost vertical slopes, are left to indicate the former gen- 

 eral level of the country. In consequence of this erosion the lignite 

 beds are exposed at frequent points and for long distances. They 

 vary in thickness from a few inches to six or eight, and even ten 

 feet, and in quality from mere carbonaceous shale to that of a texture 

 so compact and dense as to present the general appearance of cannel 

 cpal. 1 The metamorphism resulting from the combustion of these 

 beds varies, of course, in degree and extent with the thickness of the 

 burned out beds. Over immense areas, embracing hundreds and 

 even thousands of square miles, the lignite beds seem, in some cases, 

 to have been wholly consumed, all the ridges and buttes being either 

 capped or banded with the reddened, indurated shales, that have 

 resulted from the combustion. 



The metamorphosed beds consist generally of, first, a thin stra- 

 tum of grayish cinders and pumiceous matter, bearing a striking re- 

 semblance to the ashes, cinders and clinkers resulting from the combus- 



*At a point on the left bank of the Yellowstone, near the mouth of Powder 

 River, is an exposure of two heavy heels of excellent quality, separated by about 

 three feet of soft clay shale. The lower bed has a thickness of five feet, and the 

 upper of eight feet. On Custar's Creek, about ten miles above this point, a heavy 

 bed is frequently exposed, with a variable thickness of six to ten feet. Exposures 

 of this, or other heavy strata, were traced for a distance of some thirty miles, but in 

 places had been burned out. Here, in consequence of the great thickness of the 

 lignite beds, the metamorphism of the overlyiug strata extends through an unusual 

 thickness. Similar beds probably also extended throughout the extensive meta- 

 morphic districts of the Powder River Valley, to be hereafter more particularly 

 mentioned. 



