Ch. XXIV.] COAL-MEASUKES. 359 



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 in contact with the subjacent Old Red Sandstone ; but as we 

 proceed northwards to Yorkshire and Northumberland it begins to alter- 

 nate 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 contain- 

 ing 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 Yel- 

 ow Sandstone, also considered as carboniferous from its marine fossils, 

 although passing into the underlying Devonian. A similar 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 Mr. 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, (fee.) with shales and 



limestone 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 w r ater 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 as we see in the deltas 

 of great rivers in warm climates, which are liable to be submerged be- 

 neath fresh or salt water should the ground sink vertically a few feet. 



In one section near Swansea, in South Wales, where the total thick- 

 ness of strata is 3246 feet, Ave learn from Sir H. De la Beche that there 

 are ten principal masses of sandstone. One of these is 500 feet thick, 



* Sedgwick, GeoL Trans., Second Series, vol. iv. ; and Phillips, Geol. of Yorsh. 

 part 2. 



