828 



COAL 



basaltic mass, which is about 70 feet thick. Theso coal-bearing beds appear to rest 

 directly on tho mica-schist. 



' As an additional characteristic of the Irish coal-fields, there follows a summary of 

 the fossil plants which we had the opportunity of examining. I do not think that we 

 should be justified, from a consideration of these plants, to refer the coal-bearing beds 

 of Ireland to an older zone than to tho lower, generally poor, etage of the Westphalian 

 coal-formation, which represents very well the relations of their deposition. They 

 indicate here, as there also, the commencement of the Siffillaria zone, whose later 

 and richer beds were, in Ireland, destroyed, and of which only a very small area may 

 still be preserved in some northern localities, as at Dungannon.' 



The following general statement of the geological conditions of the coal-measures 

 of these islands is continued from the pen of the late Dr. Ure : 



The great Carboniferous Formations of these islands may be subdivided into three 

 orders of rocks : 1. the coal-measures, including their manifold alternations of coal- 

 beds, sandstones, and shales ; 2. the millstone grit and shale towards the bottom of 

 the coal-measures ; 3. the carboniferous limestone, which, projecting to considerable 

 heights above the outcrop of the coal and grit, acquires the title of mountain lime- 

 stone ; resting on the Old Bed Sandstone, which may be regarded as the connecting 

 link with the transition and primary rock basin in which the coal system lies. 



The coal series usually, but not invariably, consists of a regular alternation of 

 mineral strata deposited in a great concavity or basin, the sides and bottom of which 

 are composed of transition rocks. This arrangement will be clearly understood by in- 

 specting^. 466, which represents a section of the coal-field south of Malmesbury. 



Mendlp Hills 



Dtmdry Hill 



Wick Hocks Fog hill N. of Lansdowne 



1, 1, Old Red Sandstone. 2, mountain limestone. 3, millstone grit. 



4, 4, coal-seams. 6, Pennant, or coarse sandstone. 6, New Red Sandstone, or red marl. 



7, 7, lias. 8, 8, inferior oolite. 9, great oolite. 10, cornbrash and Forest marble. 



No. 1, or the Old Kcd Sandstone, may therefore be regarded as the characteristic 

 lining of the coal-basins ; but this sandstone rests on transition limestone, and this 

 limestone on highly-inclined beds of slaty micaceous sandstone which, on the one hand, 

 alternates with, and passes into a coarse breccia, having grains as large as peas ; on 

 the other, into a soft argillaceous slate. The micaceous sandstone stands bare on the 

 north-eastern border of the Forest of Dean, near the southern extremity of the chain 

 of transition limestone which extends from Stoke Edith, near Hereford, to Flaxley on 

 the Severn. It is traversed by a defile, through which the road from Gloucester to 

 Boss winds. The abruptness of this pass gives it a wild and mountainous character, 

 and affords the best opportunity of examining the varieties of the rock. 



The limestone consists in its lower beds of fine-grained, tender, extremely argillaceous 

 slate, known in tho district by the name of water-stone, in consequence of the wet 

 soil that is found wherever it appears at the surface. Calcareous matter is inter- 

 spersed in it but sparingly. Its upper beds consist of shale alternating with extensive 

 beds of stratified limestone. The lowest of the calcareous strata are thin, and alternate 

 with shale. On these repose thicker strata of more compact limestone, often of a dull 

 blue colour. The beds are often dolomitic, which is indicated by straw-yellow colour, 

 or dark pink colour, and by the sandy or glimmering aspect of the rock. 



Tho Old Bed Sandstone, whoso limits are so restricted in other parts of England, hero 

 occupies an extensive area. The space which it covers, its great thickness, its high 

 inclination, tho abrupt character of the surface over which it prevails, and the conse- 

 quent display of its strata in many natural sections, present, in this district, advantages 

 for studying the formation, which are not bo met with elsewhere in South Britain. In the 

 neighbourhood of Mitchel Dean, the total thickness of this formation, interposed con- 

 formably between the transition and mountain limestone, is from 600 to 800 fathoms. 



The mountain or carboniferous limestone is distinguished from transition limestone 



