200 THE BOOK OF NATURE STUDY 



sandstones, slates, and limestone, the fossils that are present being 

 marine types. In Ireland, the Welsh border, and Scotland, they 

 are in the form of red sandstones and marls with very few fossils, 

 and those of lacustrine and fresh-water types (Fig. 25). They 

 were formed in lakes to the north and in a sea to the south of 

 Britain. Volcanoes were active in Scotland and Devonshire 

 (Fig. 32). The chief life event is the enormous number of fishes 

 living at this time, almost all of cartilaginous type with an im- 

 perfectly ossified skeleton and an armour of bony scales or plates. 

 Giant eurypterids are also common, many entomostraca, and 

 some fresh-water mussels have been found. The marine life is 

 characterised by special types of brachiopods, and large numbers 

 of corals and crinoids. Trilobites are on the wane. The vegeta- 

 tion consists of ferns and of lepidodendra, the latter giant repre- 

 sentatives of the modern club moss. The rocks in Scotland are 

 quarried for paving flags at Caithness and Arbroath, and they are 

 also employed as building stones. Fossiliferous limestones con- 

 taining corals or brachiopods are polished and used for orna- 

 mental marbles in Devonshire. The most interesting landscape is 

 where there are hard volcanic bands, as in the Ochil and Sidlaw 

 Hills ; but generally the country is undulating, and on the Welsh 

 Borders it is covered with orchards and hop gardens. 



The Carboniferous System is the most important in Britain 

 from an economic point of view. The older rocks are generally 

 limestones (Figs. 18, 71, and 74), followed by grits, and those by 

 alternations of clay or shale, coal, sandstone, and ironstone, 

 known as the Coal Measures (Fig. 50), because they are the rocks 

 which, not only in Britain, but in Europe, Asia, and America, 

 yield the chief supplies of coal in the world. The British area 

 seems to have been in the first place a sea of considerable depth, 

 in which the limestones were laid down; the succeeding Mill- 

 stone Grit gives evidence of widespread shallowing of the sea 

 (Figs. 8 and 32) ; and the Coal Measures with their fresh-water 

 fossils and abundant plant remains, their ironstones, and the 

 association of their component strata, indicate a period of deposi- 

 tion in a great river delta which was, on the whole, undergoing 

 subsidence while being filled up with deposits. The coal seams, 

 which vary in thickness from a fraction of an inch to 20 or 30 



