406 COAL-MEASURES. [Ch. XXIY. 



The millstone-grit may be considered as one of the coal-sandstones 

 of coarser texture than usual, with some accompanying shales, in 

 which coal-plants are occasionally found. In the north of England 

 some bands of limestone, with pectens, oysters, and other marine 

 shells, occur in this grit, just as in the regular coal-measures, and even 

 a few seams of coal. I shall treat, therefore, of the whole group as 

 consisting of two divisions only, the Coal-measures and the Mountain 

 Limestone. The latter is found in the southern British coal-fields, at 

 the base of the system, or immediately m contact with the subjacent 

 Old Red Sandstone ; but as we proceed northwards to Yorkshire and 

 Northumberland it begins to alternate with true coal-measures, the 

 two deposits forming together a series of strata about 1000 feet in 

 thickness. To this mixed formation succeeds the great mass of 

 genuine mountain limestone.* Farther north, in the Fifeshire coal- 

 field in Scotland, we observe a still wider departure from the type of 

 the south of England, or a more complete intercalation of dense 

 masses of marine limestones with, sandstones and shales containing 

 coal. 



In Ireland a series of shales and slates, constituting the base of the 

 Mountain Limestone, attain so great a thickness, often upwards of 

 1000 feet, as to be classed as a separate division. Under these slates 

 is a Yellow Sandstone, also considered as carboniferous from its ma- 

 rine fossils, although passing into the underlying Devonian. A simi- 

 lar sandstone of much less thickness occurs in the same position in 

 Gloucestershire and South Wales. 



The following are the subdivisions adopted in the geological map 

 of Ireland, Constructed by Sir Richard Griffiths : 



Thickness in Feet. 



1. Coal-measures, Upper and Lower, ... 1000 to 2200 



2. Millstone-grit, - - - - - - 350 to 1800 



3. Mountain limestone, Upper, Middle- (or Calp), and 



Lower, - - - - - - 1200 to 6400 



4. Carboniferous slate, *700 to 1200 



.5. Yellow sandstone (of Mayo, &c.) with shales and lime- 

 stone, - - - ... - 400 to 2000 



COAL-MEASURES. 



In South Wales the coal-measures have been ascertained by actual 

 measurement to attain the extraordinary thickness of 12,000 feet; 

 the beds throughout, with the exception of the coal itself, appearing 

 to have been formed in water of moderate depth, during a slow, but 

 perhaps intermittent, depression of the ground, in a region to which 

 rivers were bringing a never-failing supply of muddy sediment and 

 sand. The same area was sometimes covered with vast forests, such 



* Sedgwick, Geol. Trans., Second Series, vol. iv. ; and Phillips, Geol. of York- 

 shire, Part 2. 



