xiv The Record of the Rocks 267 



which were laid down as sediment in fresh-water lakes in the 

 Devonian period, fossils of fishes clad in enamelled bone 

 and of scorpion-like creatures appear. 



347. The Carboniferous System is composed of thick 

 beds of limestone, which must have been deposited at the 

 bottom of a clear shallow sea, of sandstones laid down on 

 ancient beaches, and of shales which represent the solidified 

 mud of estuaries. The name Carboniferous comes from the 

 beds of coal which result from the decay of bark, fronds 

 and spores of club-mosses, and tree-ferns of giant size, on 

 the swampy margin of the ancient sea. Clay-beds usually 

 underlie coal-seams, and represent the soil in which the 

 carboniferous plants grew, being often full of the fossil roots. 

 The formation of coal is an interesting example of chemical 

 decomposition. The action of heat and pressure on veget- 

 able matter in the absence of air is to drive out more and 

 more of the oxygen, nitrogen, and hydrogen it contains, 

 combined with very little carbon. The following table 

 gives the average composition (omitting the ash) of dry 

 wood ; peat, which results from vegetation decaying in 

 recent formations ; lignite, a woody form of coal found in 

 tertiary rocks ; true coal ; and anthracite, which is apparently 

 derived by heating coal. It is conjectured that the final 

 product of this process is the diamond, which is pure 

 crystallised carbon. 



CARBONIFEROUS MINERALS 



Wood. Peat. Lignite. Coal. Anthracite. 



Carbon ... 50 60 67 85 94 



Hydrogen 6 6 5-5 3 



Oxygen and Nitrogen 44 34 28 10 3 



100 100 100 100 100 



The great limestone beds of the Carboniferous period are 

 composed of the remains of crinoids, mollusca, and many 

 other marine creatures. Amphibians mostly small, but some 

 of great size crawled through the marshes, but the only true 

 land animals preserved are of the nature of scorpions, insects, 

 and snails. 



