STEVENSON— INTERRELATIONS OF THE FOSSIL FUELS. 119 



The ash is extremely variable even in a single deposit. Only a 

 small proportion of the peaty material on the earth's surface is 

 good enough for fuel and a great part of that now forming is little 

 better than carbonaceous shale, with 25 to 50 or more per cent, of 

 mineral matters. At times peat is found with less ash than should 

 be expected, less than that contained in the original plants. Solu- 

 tion has made possible the removal ; potash and soda are usually 

 present, but in small quantity, the greater part having been removed. 

 Lime, iron and alumina are always present, though in exceedingly 

 variable proportions, this being due perhaps to the character of the 

 drainage area — but this suggestion is not always satisfactory. 



At many localities, the organic acids in solution have become 

 a cementing material, more or less disseminated throughout the 

 structureless mass, dopplerite in peat, Carbohumin in Schiefer- 

 kohle. In peat, it is gelatinous, but after the water has been 

 removed it is hard and insoluble. It has reached the latter condi- 

 tion in Schieferkohle. 



Resins, waxes and paraffins exist in peat, from which they can 

 be extracted by solvents. They have been derived directly from 

 the plants ; there is no reason to believe that they were formed during 

 the conversion into peat or that they were introduced from an 

 external source. 



The Tertiary Coals. 



Tertiary coals, of the ordinary types, are termed Braunkohle in 

 Germany and Austria but in France and English-speaking countries 

 they are known as lignite. The passage from peat to brown coal is 

 extremely gradual and occasionally, as indicated on preceding pages, 

 determination of the relations is merely a matter of personal equa- 

 tion. In Europe generally the complex group known as brown coal 

 has abundant points of similarity distinguishing it from the 

 Palseozoic or " stone " coals, so that, as Mesozoic coals are com- 

 paratively unimportant, the effort there has been to ascertain why 

 brown coal and stone coal are so unlike and to discover reasons why 

 the former could not be converted into the latter. But in North 

 America the condition is wholly different, for coals of all types 

 from wood-like lignite to bituminous, even to anthracite occur at 



