COAL 23 



great age, belonging to a period very remote in the 

 world's history. The rocks bearing the coal contain 

 other fossils, principally those of marine animals, 

 which are characteristic of them and of the period 

 during which they were formed, which is generally 

 known as the "Coal Measure period". There is 

 geological proof that at one time the coal seams 

 were much more widely spread over England than 

 they are at present; they have been broken up 

 and destroyed in the course of ages, by the natural 

 movements among the rocks and by the many changes 

 and processes of disintegration and decay which have 

 gone on ever since they were deposited. To-day there 

 are but relatively small coal-bearing areas, which have 

 been preserved in the hollows of the synclines.^ 



The seams of coal are extremely numerous, and 

 even the same seam may vary greatly in thickness. 

 From a quarter of an inch to five or six feet is the 

 commonest thickness for coal in this country, but there 

 are many beds abroad of very much greater size. Thin 

 seams often lie irregularly in coarse sandstone; for ex- 

 ample, they may be commonly seen in the Millstone 

 Grit; but typical coal seams are found embedded be- 

 tween rocks of a more or less definite character known 

 as the "roof" and "floor". 



Basalts, granites, and such rocks do not contain coal; 

 the coal measures in which the seams of coal occur are, 

 generally speaking, limestones, fine sandstones, and 

 shales, that is to say, rocks which in their origin were 

 deposited under water. In detail almost every seam 

 has some individual peculiarity, but the following repre- 

 sents two types of typical seams. In many cases, below 

 the coal, the limestone or sandstone rocks give place to 

 fine, yellow-coloured layers of clay, which varies from a 



1 The student would do well to read up the general geology of this very interest- 

 ing subject. Such books as Lyell's Principles of Geology, Geikie's textbooks, and 

 many others, provide information about the process of "mountain building" on which 

 the form of our coalfields depends. A good elementary account is to be found in 

 Watt's Geology for Beginners, p. 96 et seq. 



