C— GEOLOGY. 61 



Surprise is sometimes expressed that these stones should be found 

 in the seams of coal and not in the Coal Measure sandstones and shales 

 that are quarried or wi-ought in brick-yards. The reason is partly- 

 statistical. The weight, and still more the bulk, of these materials 

 extracted year by year is far less than the 250 to 300 millions of tons 

 of coal i'aised; but a yet more important reason is that no stone in the 

 coal as large as a man's head could escape detection by the collier, and 

 arousing the interest of the officials, whereas in a quarry it would 

 probably, if observed, be cast aside without notice as merely a blemish 

 in the stone of no more interest than any oi'dinary concretion. The 

 locus of the parent rock of these stray boulders is wholly conjectural, 

 but the great preponderance both in Britain and in America of quartzites 

 should furnish a clue, and the petrologist who will undertake the 

 investigation may certainly rely upon the sympathetic interest of Coal 

 Measure geologists and colliery managers. 



The Aberrations of Coal-seams. 



Having got our coal-swamp clothed with vegetation, and the coal- 

 forming materials accumulating, let us next consider the various inter- 

 ruptions of continuity and the aberrations to which it is liable. These 

 interferences may be either contemporaneous with the accumulation of 

 the materials, or, as one may say, posthumous. These categories, at 

 first sight, seem capable of easy and definite recognition, but, as we 

 shall see presently, it is not so easy as it looks. 



Faults, overthrusts, and unconformities may as a rule be classed 

 among what I have called the posthumous type of interference, though 

 in many cases true faults appear to have achieved a portion of their 

 total movement contemporaneously with the deposition of the seams, or 

 during the interval between seam and seam. An illustration of a con- 

 temporaneous fault is found at the Barrow Colliery, near Barnsley, 

 where, on the down-thrown side of the fault and parallel with it, 

 the Thorncliffe Thin Coat swells up from 3 feet to 5 feet 6 inches, and 

 carries a strip of cannel absent elsewhere in the mine. Of a fault 

 moving between seam and seam an example is furnished at Whitwood, 

 where a lower seam is thrown to the extent of 60 feet while an over- 

 lying one is unbroken. The case of a fault affecting an upper and not 

 a lower seam is noticed at Aldwarke Colliery.* Among the contem- 

 porary interferences with the coal-seams are to be accounted uncon- 

 formities, which, no doubt, occur on various scales of magnitude. Some 

 may be interpreted — as Mr. Clarke suggested for the great ' Symon 

 Fault ' of the Forest of Dean — as the denudation of a folded series ; other 

 examples would, as I shall presently show, be better explained, as 

 Prestwich explains the Symon Fault, as the erosion of a channel. 

 Prominent in this category of contemporary interferences must be put 

 the phenomena of split seams. A split seam is the intercalation into 

 the midst of the coal of a wedge of sandstone, shale, or the like, in such 

 wise that the seam becomes subdivided by intervening strata into two 

 or more seams. This phenomenon is of special practical importance 



■* Quart. Jour. Geol. Soc, vol. Ivii., p. 86. 



