ni] COAL-MEASURES. 4S 



intercalated between the Millstone grit and Carboniferous 

 limestone ; for this group of rocks the term Yoredale series 

 has been proposed. In the Isle of Man and Derbyshire sheets 

 of lava are interbedded with the calcareous sediments, affording 

 clear proof of submarine volcanic eruptions. 



N. England and Scotland. In the Carboniferous rocks of 

 Northumberland we have distinct indications of a shallower 

 sea. The regular succession of limestone strata in West 

 Yorkshire and other districts, gives place to a series of 

 thinner beds of limestones, interstratified with shales and 

 impure calcareous rocks. We have come within the range of 

 land detritus which was spread out on the floor of a shallow 

 sea. The lowest portion of the Mountain limestone is here 

 represented by about 200 feet of shales and other rocks 

 grouped together in the Tuedian series. The Upper Car- 

 boniferous limestone and Yoredale rocks of Yorkshire are 

 represented by sandstones, carbonaceous limestones and some 

 seams of coal, included in the Bernician series. Further 

 north, again, another classification has been proposed for the 

 still more aberrant succession of rocks; the lowest being 

 spoken of as the Calciferous sandstone, and the upper as the 

 Carboniferous limestone. The calciferous sandstone may be 

 compared with the lower limestone shale and part of the 

 Carboniferous limestone of England. The Carboniferous lime- 

 stone of Scotland probably represents the upper part of the 

 limestone of England and the Yoredale rocks of the Pennine 

 and other areas. 



Turning to the upper members of the Carboniferous 

 system — in the Coal-Measures, as they were called in 1817 

 by William Smith, — we have a series of coal seams, sandstones, 

 shales, and ironstones occurring for the most part in basin- 

 shaped areas. As a general rule, each seam of coal, which 

 varies in thickness from one inch to thirty feet, rests on 

 a characteristic unstratified argillaceous rock known as 

 Underclay. 



The accompanying diagram (Fig. 4) illustrates the frequent 

 intercalation of small bands of argillaceous and sandy rocks 

 associated with the seams of coal. 



