1 20 Coal-measures. 



South Wales and Somerset. They consist chiefly of 

 shales and sandstones, with marine shells and occasional 

 land-plants. The Millstone grit of South Wales is 

 comparatively unfossiliferous, but sometimes contains 

 the remains of plants, and more rarely Orthoceras and 

 other marine shells. 



The Coal-measures and Millstone grit of Monmouth- 

 shire, Glamorganshire, and Pembrokeshire, lie in a 

 great oval basin, encircled by a rim of Carboniferous 

 Limestone, beneath which lies the Old Red Sandstone. 

 The Coal-measure beds alone were estimated by Sir 

 William Logan at from 10,000 to 12,000 feet thick. 

 They consist of alternations of sandstone, shale, fireclay 

 or underclay, coal, and ironstone. There are about 1 00 

 beds of coal in the field, many of which are workable, 

 chiefly in the lower part of the series, where the prin- 

 cipal ironstones also occur. In the shales and sand- 

 stones large stems of plants are sometimes found stand- 

 ing vertically, in the positions in which they grew. 

 Underneath each bed of coal is a bed of underclay with 

 Stigmaria, forming the soil in which the plants were 

 rooted, by the decay of which, passing through the 

 stage of peat, material was supplied for the subsequent 

 production of coal. Stigmaria, once supposed to be a 

 special plant, was first proved by Mr. JBinney to be 

 the root of Sigillaria, and about the same time Logan 

 showed that the underclay was a soil that lay invari- 

 ably underneath beds of coal, and indeed that these 

 roots and rootlets are in every underclay. The plants 

 (the decay of which formed beds of coal) consisted 

 largely of gigantic club-mosses, such as Lepidodendron 

 and Calamites (Equisitacese) of various species, and 

 many other ferns, with a few Coniferse, &c. 



Passing from east to west in this coal-field, the coals 



