Geology of Coalhrook Dale. 439 



the clod and best coal, I have frequently seen it in smaller quantities. More rarely, petroleum 

 is found in cavities in the Penneystone nodules. 



Carburetted hydrogen gas is evolved in larger quantities in the deep-seated coals, and in less 

 abundance in those which are nearer the surface, and, consequently, the deep works at Dawley, 

 Malinslee, Priorslee, &c., suffer much more from this gas than the works at Lawley, Ketley Hill, 

 &c.* In the first-mentioned districts the disengagement of the gas from the pores and fissures in 

 the clod, best and sill coals, especially at the commencement of a new work, is considerable, but 

 still much less than is given off under similar circumstances in the Newcastle and some other coal 

 fields. In walking along the wall of the coal, the gas may be constantly felt issuing from the small 

 crevices. It is worthy of remark, that the evolution of the gas always greatly increases upon ap- 

 proaching the Lightmoor and other large faults ; near which masses of coal are frequently blown 

 off the bed, with loud detonations. Now, may not this larger accumulation of gas in the vicinity 

 of the faults be owing to the powerful action which accompanied their formation? 



To account for the enormous quantity of gas evolved in the Newcastle coal-field, Mr. William 

 Button made a series of interesting microscopic investigations, and ascertained that the coal con- 

 tained numerous small cells, in which he supposes the gas was originally confined in a fluid state ; 

 and that when the great superincumbent pressure is decreased in the progress of the works, the 

 elasticity of the fluid immediately causes it to assume its gaseous form. — the great and sudden ex- 

 pansion being accompanied with loud detonations f. 



Carbonic acid gas is more rarely met with, and chiefly in the old workings. Near the roof of 

 a small excavation in a pit at Caughley is a narrow opening, through which a stream constantly 

 flows, and descends to the floor, where it forms a stratum from eight to twelve inches thick, the 

 ventilation of the pit preventing its further accumulation ; but a small shaft in the same chamber 

 is completely filled with it. This great supply is probably derived from some of the neighbouring 

 old works, — the gas passing through fissures in the rocks. Carbonic acid is seldom present in 

 any quantity during the working of a pit; but in many, when abandoned, the atmospheric air is 

 soon expelled by it. 



Note. 



Coalbrook Dale has been long celebrated for its iron-works_, which once 

 produced the best iron in England ; but it is not known at what period the 

 manufactory of this metal was there commenced. From the extent of the out- 

 crops of the Crawstone and Penneystone ironstone measures, and of the little 

 flintj clod, best, sill, sulphur, and big-flint coals, no portion of the country could 

 offer greater facilities for procuring an easy and large supply of ore, flux, and 

 fuel. In many parts of the coal-field, where the necessary materials are now 

 exhausted, are large heaps of slags and cinders, for which even tradition does 

 not assign an origin;]:. 



* To the escape of this gas, from one of the deep-seated coals, through some small crack in the 

 superincumbent strata, no doubt is to be attributed the phenomenon of the celebrated burning 

 well at Broseley, discovered in the beginning of the last century, and destroyed by the sinking of 

 an adjoining coal pit in 1755. 



t Proceedings Geological Society. — Vol. i. p. 415. 



X A few years since were found in a coal pit, 80 yards deep, and on the banks of the Severn, 



