South-western Coal District of England: 293 



air is allowed to escape at the angles between the shaft arid the square pit in 

 which the shaft is constructed. 



The quantity of water discharged at the occasional piercing of these reser- 

 voirs, had led not long since to the belief, that an inexhaustible subterranean 

 river was continually traversing them; and accordingly, from the Avon to 

 the collieries of Radstock a canal was cut, for supplying which with water 

 it was calculated that this river would suffice. In a few days after the engine- 

 pumps had begun to work, the water failed ; since, after exhausting the reser- 

 voir, an additional supply could only be derived from the slow percolation of 

 rain-water through the incumbent strata. The canal has in consequence be- 

 come useless, and a rail-road has been substituted for it. 



The conglomerate is often metalliferous, yielding occasionally some galena, 

 and more frequently calamine, in some abundance. These ores occur some- 

 times in small veins, but are usually disseminated through the rock, particu- 

 larly when it is charged with calcareous spar. The galena, when thus dis- 

 seminated, occurs in angular fragments ; it appears pretty extensively in this 

 form in the compact, yellow dolomite of Old Clevedon and Portishead. 

 Much of the calamine has been formed stalactitically around dog-tooth spar, 

 and is now found in the shape of hollow casts and pseudomorphic cry- 

 stals, in consequence of the calcareous matter having perished. These ores 

 are probably of simultaneous origin with the conglomerate itself, having 

 been derived mechanically from the debris of metalliferous veins which tra- 

 versed the mountain limestone. We may be induced however to hesitate in 

 adopting this theory, by reason of the metalliferous character of the Zechstein 

 and Alpine limestone, the geognosfic equivalents on the continent of the 

 dolomitic conglomerate of England. 



Sulphate of strontia occurs abundantly in the conglomerate in the vale of 

 Westbury. The thickness of the conglomerate is very variable, often chang- 

 ing suddenly from many fathoms to a few inches, as might naturally be ex- 

 pected in a stratum which was once an accumulation of derivative graved 

 Thus it generally forms a thick bank or talus near the base of those hills 

 from whose debris it has been derived ; while at a distance from them it 

 grows thinner, and at length wholly disappears. Sometimes it hangs on the 

 sides of the mountain chains at considerable elevations, covering their trun- 

 cated escarpments or posterior slopes ; occasionally it fills up partial depres- 

 sions in the high table-lands at the very summits of those chains. Not un- 

 frequently, when the talus-shaped strata haye been deeply furrowed by dilu- 

 vian action, the remarkable appearance has resulted of. large insulated masses 

 of conglomerate, resting unconformably, at irregular elevations, on the 



