THEIR COMPOSITION. 163 



pure, and spread over a considerable area, with great regu- 

 larity as to thickness and quality. Surprise has been fre- 

 quently expressed at the uniform thickness which many 

 coal-seams maintain over extensive areas. Growth of the 

 vegetable mass in situ is no doubt the main cause, but we 

 must not lose sight of the fact that, when in a semi-plastic 

 state of bituminisation, the pressure from above would have 

 a tendency to spread out the seam, and insensibly equalise 

 its thickness. On the other hand, where interruption to 

 this growth has taken place either from periodical inun- 

 dations or otherwise, the seam contains layers of earthy 

 matter, and is more or less impregnated throughout with 

 such impurities. Again, where the seam has arisen from 

 drifted vegetation, it is still less regular in thickness, and 

 often so earthy and impure as to pass into a bituminous or 

 coaly shale. These bituminous or coaly shales, now coming 

 so largely into use in the Scottish coal-fields for the distil- 

 lation of paraffin and paraffin oil, are indeed of very vari- 

 ous origin and composition. Some of the richer sorts are 

 merely compressed and mineralised vegetable muds that 

 have arisen from long maceration and decay; others, during 

 this long decay in shallow water, have got so largely mingled 

 with the remains of minute crustaceans (cyprides, &c.), as 

 to be partly of animal origin ; * and some again of the poorer 

 sorts are little else than thick clayey silts, irregularly inter- 

 mingled with vegetable and amimal debris. In this way 

 the various purities of coal can be readily accounted for, 

 while difference of mineral aspect and quality may have 

 arisen partly from the nature of the vegetation, partly from 



* One of the most remarkable we have examined is the " Grey Shale " 

 of West Calder, raised for the extensive paraffin works of Mr Young, 

 and which derives its name from the colour imparted to it by the cal- 

 careous cases of these minute organisms. In some places the seam, 

 which is upwards of two feet thick, is literally a mass of these remains. 





