496 Henry Woodward — Old Land- surfaces. 



In England it attains a thickness of 1500 feet, whilst in Ireland its 

 mass is even greater still. 



Above the Carboniferous Limestone comes a coarse qiiartzose sand- 

 stone, containing but few fossils, and known as the " Millstone Grit" 

 (from the economic use to which its material is applied). This 

 deposit is above 600 feet in thickness, but is not purely of marine 

 origin. 



Again, above the Millstone Grit come the "Coal-measures" proper. 



Some idea of their vast geological importance may be formed 

 when we find that in the South Wales Coal-field the strata attain an 

 aggregate thickness of 12,000 feet ! 



The Coal itself forms only a small proportion of this mass, the 

 main part being made up of intercalated beds of mixed freshwater, 

 and marine origin, and comprising layers of shale, sandstone, grit, 

 clay, and ironstone. 



Wherever the Coal-measures have been examined, beneath each 

 seam of Coal is found a layer called the "under-clay" or "fire- 

 clay," which forms the floor upon which the Coal itself rests. 



Every one of the 100 seams of Coal in the South Wales Coal- 

 field has its under-clay. These Clays, full of roots of plants (called 

 Stigmaria), are, in fact, the soils in which the trees and plants grew, 

 which formed by their growth and decay the several layers of Coal. 



In many instances the trunks of these old fossil trees have been 

 found standing erect, still attached by their roots to the soil ; their 

 decayed and hollow trunks (filled up from above with sediment 

 within) and often penetrating through several of the superimposed 

 layers by which the Coal-seam is covered. 



Indeed, these aged trees have proved of the highest geological 

 importance, for within their hollow walls have been found a Myria- 

 pod, two land-snails {Pupa vehista said. Zonites prisms), and several 

 species of small Eeptilia, which, either intentionally or by accident, 

 had found their way in, but, being unable to escape, they were en- 

 veloped in stone, to remain hidden until the energy of Dr. Dawson, 

 of Montreal, should bring them to light. 



Coal is, as already stated, the product of the destruction of the 

 continued growth of plants in situ. The more perfect the chemical 

 conversion of the tissues of plants into Coal, the less are we able to 

 detect the presence of the organisms which have contributed to its 

 formation. Nevertheless, the " mother-coal," which occurs between 

 the layers of completely -formed Coal, is composed of the broken-up 

 tissues of the plants converted into Anthracite, but still retaining 

 their external forms. 



Prof. Morris originally pointed out that the " better-bed-coal " 

 owed its peculiar chemical composition, which gave it its great 

 value for smelting purposes, to the fact that it was composed entirely 

 of a mass of spore-cases, which Mr. Carruthers has shown to belong 

 to a Lepidodendroid genus called Flemingites (See Geol. Mag., 1865, 

 Vol. II., PI. XII., p. 433), and (as he informs me) to Sigillaria, and 

 other allied forms. Prof. Huxley subsequently made the interesting 

 observation, from specimens prepared with singular care and skill by 



