GEOLOGY. 49 



continuous over the whole area; although that they were once so is 

 more than probable. 



This deep furrowing of the land, which is common more or less to 

 every coalfield in the island, has been ascribed by geologists to the action 

 of great floods, at a period antecedent to all human records, carrying 

 along with them gravel and blocks of stone, which have ploughed up the 

 ground, and borne off the loosened materials, to be afterwards deposited 

 in different parts, leaving behind them extensive valleys. The effect of 

 this action has been called denudation by geologists, and the valleys 

 so formed, which are not peculiar to coal fields, but exist in many parts 

 of England, are called valleys of denudation. No bed of coal is 

 uniformly good throughout any great extent ; the high main coal is for 

 many miles so deteriorated in quality, and so mixed up with stone, that 

 it becomes worthless in many places. The coal-seams worked on these 

 fields vary from 18 inches to 14 feet in thickness; but in the thick seams 

 there is always a considerable portion of such bad quality as not to be 

 saleable at a profit, and the best quality is seldom more than 6 or 7 feet 

 thick. The best beds are those called by miners the high main and the 

 low main; and, deep as the latter is, it is considered as quite a superior 

 bed. 



MODE OF WORKING COAL MINES. 



No instances occur in this country of beds of coal lying so near the 

 surface, that they can be worked in open day like stone quarries, nor are 

 they often found on the side of a hill so that they can be worked 

 horizontally. When a coal field is to be won, that is worked, the first 

 step is to sink a perpendicular, circular shaft, like a great well, in order 

 to get at the coal, and by which the miners or pit- men descend, and the 

 coal is brought to the surface. The sum required for -winning a bed of 

 coal is so great, that it is usually done by a company of speculators. One of 

 the difficulties in sinking a shaft is, passing through quick-sands; another 

 is the immense quantities of water which are met with in certain parts 

 of the stratification, generally within 40 or 50 fathoms of the surface, 

 which is always damned back by a tub : the depths of the mines vary 

 very much ; in one place near Jarrow, five miles from the Tyne, he high 

 main of coal is found 42 feet under the surface, while at Jarrow lake it is 

 1 ,200 feet from the ground. This great depth is not reached by one 

 perpendicular shaft, but by a shaft and steam engine under ground, witn 



