ig8 THE BOOK OF NATURE STUDY 



In what follows an attempt is made to give a brief description 

 of the successive rock Systems, so as to bring out their broader 

 characters. What is said with regard to the life is not confined 

 to the British Isles, but any description of the physical character 

 of the rocks will necessarily apply chiefly, and often exclusively, 

 to the British Islands. 



The rocks of the Eozoic Group are only seen in limited localities 

 where newer rocks have been stripped off. They are usually 

 much altered from their original condition, and are in the form 

 of gneisses and schists. Associated with these are many igneous 

 rocks, either poured out as lavas (with their accompanying tuffs 

 and ashes) or intruded into others at various depths. There are 

 also sediments, conglomerates, felspathic grits, and slates, with 

 rare limestones. In the sediments have been found traces of 

 worms and a few other organisms. The rocks are found some- 

 times in great plateaux, at others along the core of modern moun- 

 tain chains or of more ancient chains which have now been 

 deeply cut into by denudation. The rocks yield rich supplies 

 of metals such as iron and copper, gem stones, marbles, and stones 

 for building and roofing. The landscape on their surface is some- 

 times undulating and monotonous, but at times mountainous and 

 beautiful. 



The Cambrian Rocks are typically seen in Wales. They 

 consist of slates, quartzites, and grits, the deposition having taken 

 place in deep seas only shallowed occasionally and over a limited 

 area. The life of the period consisted mainly of trilobites and 

 horny brachiopods, with few lamellibranchs and gastropods ; 

 cephalopods, crinoids, and phyllopods have been found. The 

 rocks yield slate and road metal, manganese and gold locally. 

 Their landscape is not usually of much interest, but the great 

 exception is the Harlech and Tremadoc region in Wales, where the 

 harder bands form lofty mountains of very massive structure. 



The Ordovician Period was one of deep seas sprinkled with 

 volcanic peninsulas and islands. The rocks are fine shales or 

 slates, with flagstones and limestones, generally interbedded with 

 volcanic ashes and lavas, and having innumerable masses of igneous 

 rock intruded into them. Some of the rocks, like the intensely 

 fine-grained shales, showing an apparently rapid life-succession, 



