410 E. S. BASTIN 



other portions, especially near the slaglike masses (No. 3, below), 

 are extensively and very irregularly fractured. The metamorphism 

 of these rocks finds a parallel in the artificial process of the burning 

 of bricks, and consists simply in an oxidation of the iron from the 

 ferrous to the ferric state, and in incipient fusion which greatly 

 increases the coherence. 



2. The coal itself leaves behind a typical ash upon burning; 

 most of this is loose and incoherent, but some parts are clinker-like. 



3. Slaglike masses make up only a small proportion of the beds, 

 but are striking because of their close resemblance to lavas. They 

 form very irregular, dark brown or mottled, vesicular masses, and 

 usually inclose numerous fragments of shale and sandstone. Occa- 

 sionally the slag occurs as an uneven layer just above the burned-out 

 coal bed, but in most of the localities visited by the writer it formed 

 isolated masses which in many instances seemed to be almost sur- 

 rounded by the baked clays described above. Allen 1 also describes 

 "chimney-like" forms a few feet in diameter capping buttes because 

 of their superior resistance. The forms of the slaglike masses, and 

 the sharp transition to beds which have been but slightly metamor- 

 phosed, suggest that they represent channels of easy exit for the hot 

 gases and vapors, along which the metamorphism was much more 

 considerable than in any other portion, except immediately above 

 the coal bed, where slags are also occasionally developed. The ropy 

 surfaces exhibited by some of the slags show how fluid much of the 

 material must have been. 



4. The inclosing argillite near the slaglike masses has in most 

 cases been much brecciated, and one of the characteristic types of 

 rock is formed by the penetration of the crevices of such breccia by 

 the slag. The process usually results in the induration of the breccia 

 fragments to hard, flintlike masses, red, buff, or gray in color. 



A microscopic examination of one of these slag veins in red argil- 

 lite showed a central, relatively coarsely holocry stall ine mass about 

 one-tenth of an inch across. This was bordered on each side by a 

 zone about one-fiftieth of an inch wide, fine-grained, prevailingly 

 gray in color, and apparently representing the contact effect of the 

 slag on the argillite. Outside this narrow zone the argillite shows 



1 Ibid., p. 250. 



