Coal-measures. 



121 



(sometimes the very same beds) gradually change 

 from so-called bituminous to anthracitic varieties. It 

 is remarkable that anthracite usually occurs in coal- 

 fields the strata of which have been much disturbed 

 and contorted, as, for instance, in the mountains of 

 Pennsylvania. Anthracite is simply a metamorphic 

 variety of coal ; and in Pembrokeshire, where the coals 

 are most anthracitic, the strata have been violently 

 contorted. There is a connection between the heat 

 that produced metamorpljism and the lateral pressure 

 that produced contortion, for pressure with movement 

 is convertible into heat. A line of disturbance passes 

 from the banks of the Wye south of Builth, through 

 the north part of the coal-field south of Llandeilo, and 

 from thence westward into Pembrokeshire, where masses 

 of igneous rocks appear in contact with the coal-field. 

 In connection with this, it may be that the rocks of the 

 coal-field remained a long time highly heated, and so, 

 by a species of distillation, deep under ground, the 

 bituminous were converted into anthracite coals. 

 FIG. 27. 



1. Old Red Sandstone. 



2. Carboniferous Limestone. - 



3. Millstone Grit. \ Carboniferous series. 



4. Coal measures with beds of coal. J 



Dean Forest may be looked on as an outlier of the 

 South Wales coal-field. Fig. 27 may be supposed to 

 represent the arrangement of the strata on the east side 

 of this very perfect basin. The limestone is about 700 



