Allen.] 262 [January 21, 



earth, as has happened in the great coal fields of Pennsylvania, of 

 England, Germany and elsewhere. Although it seems almost impos- 

 sible that a sufficient supply of oxygen to support combustion could 

 reach these fires, it is well known that they are in reality exceedingly 

 difficult to extinguish, and that they will smoulder for years when every 

 effort has been made to smother them by closing every communica- 

 tion with the external air. I have, however, met with no account of 

 any extended metamorphic action attending these burning coal, seams 

 such as always attends the burning of the lignite beds of the Upper 

 Missouri country. This difference is doubtless owing to the different 

 condition of the enclosing shales of the true coal and the lignite beds. 

 In the former the strata are usually already indurated, and are not 

 readily modified by heat ; in the latter they consist of soft clays and 

 sands, which are easily influenced by heat, speedily becoming baked 

 and reddened when exposed to great heat, like the brick-clays when 

 subjected to heat in a brick-kiln. As an illustration of how readily 

 these clays become indurated by even moderate heat, I may mention 

 a familiar incident of camp life. Owing to the violence of the winds 

 in the region under consideration, it is often necessary to build the 

 camp-fires in little pits to protect them from the wind, when the 

 walls of these pits, after exposure to the fire for but a single day, 

 become indurated to a considerable depth. 



An apparently similar combustion of coal-seams in the Celestial 

 Mountains has been described by Semenof, 1 and in the mountains of 

 the Upper Zaraphan and the Tian Shan ranges by Severkof, 2 the 

 earlier, vague descriptions of which led Humboldt to suppose these 

 regions were seats of volcanic action. 



The influence of the metamorphism above described upon the to- 

 pography of the country where it occurs is by no means slight. Not 

 only do the baked, indurated clays and sands give their own preva- 

 lent bright red tint to the landscape, but they arrest or greatly retard 

 the erosion of the buttes and ridges whose summits they compose. 

 Over areas of thousands of square miles in extent they thus in great 

 measure determine the surface contours, and protect the hills from 

 an otherwise rapid demolition by the agency of aqueous denudation. 



1 Journ. Roy. Geog. Soc, Vol. xxxv, p. 213. 



2 Ibid., Vol. xl, pp. 395, 396. Also quoted in Nature, by Howarth, Vol. ix, p. 142. 



