GEOLOGY 



thickness when compared with the enormous mass of rapidly alternating 

 sandstones and shales with which they are interbedded, they are yet much 

 more constant than any of these, and the accurate knowledge of them 

 derived from the innumerable spots at which they are, or have been, 

 worked throughout the coalfield gives them a commanding position as 

 stratigraphical units such as no other deposits associated with them can 

 claim. It is not necessary here to enter into the interesting, and at the 

 present day rather controversial, question of the origin of coal generally, 

 especially as the seams of Durham are most of them of a kind which 

 does not give rise to much difference of opinion. With very few ex- 

 ceptions these seams, each provided with its regular seat-earth or'undcr- 

 clay ' — which is also almost in every case a fire-clay — are obviously 

 accumulations of vegetable matter in low-lying swampy flats of great area, 

 and most of this vegetable matter is doubtless in its carbonized or coaly 

 state much in the place where it grew and flourished when living ; the 

 under-clays in which the strange tree-roots known as Stigmarice are 

 found quite undisturbed representing the soil beneath the heaped up de- 

 cayed plant remains of the watery marsh. That these plants, some of 

 them gigantic in size, were chiefly allied to the club-mosses, horse-tails 

 and ferns of the present day is clear from the many well-preserved speci- 

 mens which not the coals themselves but the shales and other beds 

 accompanying the coals yield throughout the Coal Measures. The 

 animal remains which are also, though less often, found tell the same 

 tale. They are the exuvia; of fishes whose rare recent allies inhabit 

 fresh or at least estuarine waters, of alligator-shaped amphibia fitted for 

 similar conditions, and of shells (chiefly bivalves) which apparently lived 

 the life of our river and pond mussels. Occasionally some of the animal 

 forms are consistent with existence in brackish waters, but instances of 

 frankly marine forms such as those which obtain in the Carboniferous 

 Limestone Series, though not absolutely unknown, are yet of great 

 rarity, and suggest, when they do occur, brief episodes only during which 

 quite occasional incursions of the sea may have invaded the delta-like 

 swamps. 



The Durham coals are almost all of the ordinary or so-called ' bitu- 

 minous ' type and furnish some of the best examples of household, cok- 

 ing and gas coals known. A few deposits of cannel coal occur, but they 

 are all of very limited extent and small thickness. They moreover as 

 a rule form part of the ' bituminous ' seams, occurring usually towards 

 the upper portions of such seams over small areas. Microscopic exam- 

 ination shows that these sporadic cannel beds (which sometimes are 

 locally thick enough for working separately, and then yield gas of ex- 

 ceptional illuminating power) largely consist of minute freshwater alga 

 which lived, presumably, in shallow pools dotted here and there upon 

 the surface of the forest swamps. True anthracite is not found in the 

 county, though as a trade term the use of the word ' anthracitic ' is not 

 unknown in prospectuses describing coals with a somewhat smaller pro- 

 portion of volatile matter than is usual in the common coals. Some- 



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