120 STEVENSON— INTERRELATIONS OF THE FOSSIL FUELS. 



times within a .single district, in a single bed or even within the 

 limits of a single estate. The passage from one type to the other 

 is so gradual that chemists and geologists of North America have 

 labored to discover some means of distinguishing them. The prob- 

 lem is no longer one of merely abstract or scientific interest ; it is 

 of the utmost practical importance, since within a vast area the only 

 source of supply is in the Tertiary and Upper Cretaceous deposits. 

 The effort is to determine distinctions which will be available for 

 both the seller and the purchaser of fuel. 



In most works, the characteristics of brown coals are given 

 definitively. Though in mass the color may be black, yet the 

 powdered material has a brown tint; the content of water is con- 

 siderable and air-dried specimens retain lo per cent, or even much 

 more, so that brown coals have been termed hydrous; jointing or 

 cleat is wanting or at best ill-defined ; the water in air-drying escapes 

 through shrinkage cracks or along bedding planes and the coal falls 

 into small fragments ; brown coals do not coke ; solution of caustic 

 potash attacks brown coal and acquires a reddish or brownish tint ; 

 brown coals contain more carbon than peat but notably less than 

 the stone coals. 



These features are characteristic of brown coals in general but 

 they are not wholly distinctive. Some brown coals yield a black 

 powder and some Carboniferous coals give a brownish powder; 

 many Carboniferous coals retain more than lo per cent, of water 

 and there are Tertiary coals with very much less ; there are Tertiary 

 coals with very distinct cleat while there are Carboniferous coals in 

 which cleat is very indefinite; there are Carboniferous coals in ex- 

 tensive areas whose included water escapes along the bedding planes 

 and the coal breaks first into slabs and then into small fragments ; 

 a very great proportion of Carboniferous coals do not cake while 

 there are Tertiary coals which yield good coke ; caustic potash solu- 

 tion attacks many Carboniferous coals ; the carbon-content is not 

 definite. In fact, the passage from brown to stone coal is as grad- 

 ual, chemically, as that from the growing layer of peat to brown coal. 



A proper examination of this matter will be in place only after 

 study of the Palaeozoic coals. For present purposes, the classifica- 



