50 



THE BRITISH ISLES. 



80 feet, are beinj,^ worked, and the quantity of coal which it is possible to extract 

 without descending to a greater depth than 4,000 feet is estimated by Vivian 

 and Clark at more than 80,000,000,000 tons. In the west the seams yield 

 anthracite, but in proportion as we proceed eastward the coal becomes more and 

 more bituminous, the gases enclosed in it often giving rise to fearful explosions, 

 the frequent recurrence of which is a calamity which might generally be obvi- 

 ated by judicious cautionai-y measures. So fiery is some of this Welsh coal, that 

 after having been placed on shipboard it will ignite spontaneously. 



The researches of men of science have conclusively proved that Wales, within 

 recent theological time, has undergone variations of level. Marine shells of living 

 species were discovered as long ago as 1831 near the summit of Moel Tryfuen, 



Fig. 23.— EiiosivE Action on the Coast of Sot'th Wales. 

 Scale 1 : 100,000. 



W.dPGr 



Granife or 

 Syenite. 



Volcinic 

 Kocks. 



Lower 

 Silurian. 



Devonian 

 Sandstone. 



Millstone CnrVioniferous Coal 

 Giit. Limestone. Measures. 



10 Miles. 



to the south of the Menai Strait, at an elevation of 1,400 feet above the 

 level of the sea. This discovery has been confirmed and followed np by other 

 geologists, including Edward Forbes, Prestwich, Ramsay, Darwin, and Lyell. 

 Mr. Darbishire has found fifty-seven marine molluscs in the upheaved strata 

 which during the post-pliocene epoch formed the beach, and all these shells belong 

 to species which still live in the neighbouring sea or in the Arctic Ocean. The 

 general character of this ancient fauna points to a climate as rigorous as that of 

 Iceland or Spitzbergen. The British seas were colder at that time than now, 

 and when the land once more emerged from the sea these shell banks became 

 covered with the detritus brought down by glaciers.* 



* Lyell, " Elements of Geology." 



