358 PALAEOZOIC SYSTEM OF ROCKS. 



conclude, then, that high heat is not necessary to produce anthracitism ; 

 for, if it is unnecessary for metamorphism of rocks, much less is it neces- 

 sary for metamorphism of coal. 



Plants of the Coal — their Structure and Affinities. 



The flora of the coal-measures is one of the most abundant and per- 

 fect of all extinct floras ; according to Ward, there are about 8,660 

 known fossil species of plants, and of these about 2,000, or nearly one 

 fourth, are from the coal-measures.* This flora is peculiarly interest- 

 ing to the geologist, not only on account of its relative abundance, but 

 also and chiefly because, being the first diversified and somewhat 

 highly-organized flora, it is natural to suppose that the great classes 

 and orders of the vegetable kingdom commenced to diverge here ; and 

 therefore it furnishes a key to the evolution of land-plants. We will, 

 therefore, discuss the affinities of these plants somewhat fully. 



Where found. — The plants of the Coal are found principally : 1. In 

 the form of stools and roots in their original position in the under-clay ; 

 2. Of leaves, and branches, and flattened trunks, on the upper surface 

 of the coal-seam, and in the overlying shale ; 3. And, finally, in the 

 form of logs, apparently drift-timber, in the sandstones above the coal- 

 seam. The black shale overlying the seam is often full of leaves and 

 fronds of ferns, and of the flattened trunks of other families, in the 

 most beautiful state of preservation, so that even the finest venation of 

 the leaves is perfectly distinct. In some cases where the shale is light- 

 colored, so as to contrast strongly with the jet-black leaves, the effect 

 on first opening a seam is very striking, and has been compared to the 

 frescoes on the ceilings of Italian palaces. 



Principal Orders. — Leaving out some plants of doubtful affinity, 

 the plants of the Coal may be referred to five orders or families, viz., 

 Conifers, Ferns, Lepidodendrids, Sigillarids, and 

 Calamites. It is usual to refer these last three to the 

 two orders Lycopods and Equisetse ; but they are so 

 peculiar, and their affinities still so doubtful, that we 

 have preferred to treat them as distinct orders. 



All these, as already seen, commenced in the De- 

 vonian, as did also the preservations of their tissues as 

 coal ; but both the vegetation and the conditions 

 necessary for their preservation culminated in the 

 Coal period, and therefore we have put off their dis- 

 cussion until now. Contrary to our usual custom, we 

 fig. 458.-Trunk of a w {\\ commence with the highest, viz. : 



Conifer: a, bark; o, ° 7 



wood; c, medullary J, Conifers. — A considerable number of genera of 



sheath ; d, pith. ° 



* Science, vol. iv, p. 340, 1884. 



