Geology of Coalhrook Dale. 433 



precisely with the undisturbed measures of Donnington, but all the coal and 

 ironstone above that stratum had vanished. (See shaft section. Diagram, PI. 

 XXXVII.). 



The surfacesofsome of the strata are furrowed andwaved.and saddle-shaped 

 ridg-es, two to four feet in height and several yards in length, sometimes exist 

 on the surface of the shales underlying the coal-beds; but they gradually dis- 

 appear in the overlying strata. In the sandstone, especially in the coarser beds, 

 the under surface is in places very uneven, filling up hollows and grooves in the 

 subjacent shale or coal, and resulting apparently from the abrading action of 

 the water which deposited the sandstone. In the shale roof of some coals 

 are occasionally found singular misshaped conical masses, which from their 

 smooth surface fail out, to the annoyance of the workmen, as soon as the sup- 

 porting coal is removed. They are traversed by divisional and slightly con- 

 cave planes at right angles to their axes, but otherwise exhibit no external or 

 internal structure. They are formed of the same substance as their matrix, 

 from which they are sometimes separated by a thin parting of coal. The 

 name given to them is " pot-holes " or " hatter-blocks." Their height is 

 generally two or three feet, and the diameter at the base equals one-third or 

 half of the height. 



Shales, sometimes slightly micaceous, are not of frequent occurrence, but 

 hard indurated clays are common, and often contain ironstone nodules. The 

 argillaceous strata predominate in the north of the coal-field, and sandstones 

 in the south. The latter are occasionally micaceous and fissile, and some- 

 times form thick-bedded freestones, but at other times are coarse-grained 

 and conglomeratic. The imbedded pebbles almost invariably consist of 

 milk-white quartz, but a few of a red colour are associated with them, and 

 the matrix apparently consists of the same materials finely comminuted. 

 Partings, filaments, and roundish lumps of coal, are common in the upper 

 sandstones. Petroleum impregnates many of the central sandstones ; and salt- 

 water pervades some of the sandstone strata. Dr. Townson says, "In the 

 neighbourhood of this tar spring (at Madeley), several springs of salt-water 

 have been found, but none of sufficient strength to be worth working. The 

 water which exudes from the flint coal in some works in the parish of Madeley, 

 is likewise salt ; and in the adjoining parish of Broseley, there was formerly 

 a salt-work, where salt is said to have been made from water taken out of 

 the coal-pits*." 



Nos. 34. and 51. in the section of the Hill's Lane pits, are beds of an im- 

 pure calcareous conglomerate ; but as they are nowhere exposed, and were 



* Tovvnson's Tracts, p. 1 76. 



