164 THE OLD COAL-MEASUEES. 



the rapidity with which it was entombed, partly from the 

 porous or retentive character of the imbedding strata, and 

 partly also from the degree of mineralisation the respective 

 seams have undergone. Hence the soft caking coals, which 

 fuse together in burning ; the hard, slaty, splint coals, which 

 burn dry and open ; the coarse cubic coals, which also burn 

 open and leave much earthy ashes; the compact, lustrous, 

 cannel coals, used chiefly in the making of gas, and other 

 varieties well known in one or other of our British coal- 

 fields. There is no great difficulty, we repeat, in account- 

 ing for the varieties of palaeozoic coals, if we only make 

 allowance for difference in the nature of the vegetation, in 

 the modes of its accumulation, the length of time it was 

 exposed to maceration and decay, the retentive character of 

 the covering stratum, and the intensity of mineralisation 

 which these different conditions would induce. Of course, 

 all this implies long ages of growth and decay, repeated 

 emergence and submergence of the land, but in the main a 

 gradual subsidence to permit that vast accumulation of sedi- 

 ments sandstones, shales, ironstones, and limestones, to 

 the thickness of many thousand feet which constitute the 

 bulk of the Carboniferous system. 



Besides the strictly sedimentary strata, the coal-measures 

 are also in some districts largely made up of igneous pro- 

 ducts, which intermingle with them as masses of basalt and 

 greenstone, beds of trap-tuff, and other vulcanic discharges. 

 Of course, these discharges must have taken place in or 

 near the seas of deposit, now as overflows of lava, now as 

 showers of ashes, and again as the mingled products of vol- 

 canic eruption. Just as insular and submarine volcanoes 

 are at the present day mingling their eruptive matters with 

 the sediments of the adjacent seas, so in the old coal period 

 similar agencies were at work, and the results are now the 

 interstratified greenstones and trap-tuffs, the bent and frac- 



