106 THE STORY OF THE EARTH. 



bursts are comparatively rare in the coal of Eng- 

 ■i. In the Edinburgh coal-field the volcanic 

 eruptions are the most impressive feature of the 

 deposit. The layers of volcanic ash, and vesicu- 

 lar lavas, such as may be seen in Edinburgh, at 

 Calton Hill, prove the outbursts to have been 

 contemporaneous with the sediments. Regions 

 of volcanic activity are commonly ti e of 



changes of level of land, such as the coal strata 

 demonstrate. 



The layers of coal may be compared to the 

 growth of peat over the flat lands of Holland. 

 The sea sometimes bursts in, as when the Xuyder 

 Zee was formed, so that marine beds with marine 

 shells, rest upon the terrestrial growth. Such 

 catastrophes occurred in the Dudley coal-field, 

 and more evidently in that of Coalbrook Dale. 

 In the lower part of the coal measures is the 

 layer of clay ironstone known as the Pennystone; 

 and in the upper part is the layer known as the 

 Chance Pennystone ; both of these are marine de- 

 posits with marine fossils, like the shells Gomatitcs 

 and Aviculopectetly which had not been seen since 



the ( larboniferous Limestone \\ r as deposited. 



They were still existing not far away; but might 

 have been thought extinct, but for these incur- 

 sions < >f the sea. 



There is no means of judging whether the 



coal or the intervening sediments occupied the 



jer tune in forming. The total thickness of 



the coal in all the seams added together, \arics 

 from about ioo to I 40 feet. 



The properties of coal probably vary with the 



nature of the juices Of the living plants. Thus, 



w \w\\ starch is burnt it gn es .1 vesicular 1 

 cinder. When gum arable is burned, it forms a 



