South-western Coal District of England. 291 



§ J. The dolomitic conglomerate *. 



The immediate covering- of the older rocks is usually a conglomerate^ com- 

 posed of fragments of those rocks united together by a cement^ which in most 

 cases consists of dolomite^ or dolomite blended with carbonate of lime^ but 

 not unfrequently of carbonate of lime only^ of clay^ or of fine iron-shot sand. 



The fragments vary in size, from the finest grains to bowlders of 3 feet 

 and upwards in diameter, and are for the most part rounded ; though some- 

 times very slightly so, when the parent rock in situ is close at hand. The 

 fragments vvhich it contains in the greatest number, are those of the inclined 

 strata, the nearest adjacent. Thus in the overlying conglomerate, which 

 skirts the Quantoc hills, the most abundant pebbles are those of the transition 

 rocks, of which those hills consist; and throughout the coal-district here 

 treated of, when the principal surrounding chains consist of mountain lime- 

 stone, fragments of that limestone predominate ; but we also find fragments 

 of old red sandstone, and quartzose pebbles, derived probably from the older 

 conglomerates in the old red sandstone or millstone grit. Fragments of coal- 

 sandstone are extremely scarce, as might be expected from their friable tex- 

 ture. This conglomerate has evidently been spread over the surface, as it 

 formerly existed, in the state of loose gravel, like the diluvian gravel, which 

 at a later period has been scattered over the earth. The conglomerate oc- 

 casionally becomes fine-grained, and so charged with the matter of the cement, 

 that it passes into a tolerably compact dolomite, bearing no traces of mecha- 

 nical origin, and then closely agrees in character with the yellow magnesian 

 limestone of the north-eastern counties of England f. Beds of yellow dolo- 

 mite, thus strongly characterized, are found prevailing over tracts of consi- 

 derable extent, particularly at Portishead-point and beneath the ruined 

 church of Old Clevedon on the shores of the Bristol Channel, in the low 

 cliff to the north of the Old Passage-house on the right bank of the Severn, and 

 on the north-western and northern limits of the Bristol coal-basin, at the vil- 

 lages of Almondsbury and Tortworth. When we consider how abundantly 

 dolomite occurs in mountain limestone, and that in the district here treated 



* This conglomerate has been ably described in the 4th volume of the Geological Transactions 

 by Dr, Bright, Mr. Warburton, and Dr. Gilby. Mr. Warburton has most correctly considered 

 it as equivalen* to the yellow magnesian limestone of the north-eastern counties of England. 



f This rock, as it occurs in Nottinghamshire and in the north-eastern counties, being the first 

 in which the late Smithson Tennant discovered the presence of magnesia, has usually been 

 termed, par excellence, the magnesian limestone. In England, however, so many of the older 

 calcareous formations are occasionally dolomitic, and on the continent so many of the more recent 

 ones, that the term has become equivocal, unless we connect with it in each particular case the 

 name of the formation to which it belongs. 



