ORIGIN OF COAL 413 



Hardly more than 2% of the thickness of the coal measures 

 consists of workable coal. The strata are mostly sandstones, 

 shales, clays, and in some regions limestones, interstratified with 

 numerous seams of coal of very varied thicknesses. This alter- 

 nation of coal with mechanical deposits does not necessarily, or 

 even probably, imply oft-repeated oscillations of level, but may be 

 explained by assuming a general, slow, but intermittent subsidence. 

 After each submergence, we may suppose, the movement was 

 nearly or quite arrested, and the shallow w T ater was filled up with 

 sediment, until a bog could again be formed. Doubtless, move- 

 ments of elevation also occurred at times, but the general move- 

 ment was downward. In the Nova Scotia field are 76 distinct 

 coal seams, each of which implies the formation of a separate bog. 

 Beneath most coal seams occurs what miners call the " seat-stone " 

 or "underclay," which is ordinarily a fire-clay, or it may be 

 siliceous, but is always evidently an ancient soil. The underclay 

 is filled with fossil roots, from which often rise the stumps of trees 

 that penetrate the coal seam, or may even extend many feet above 

 it. The rock which lies on a coal seam is usually a shale, stained 

 black by organic matter, but may be a sandstone or even a lime- 

 stone, according to the depth of water over the submerged bog. 



That coal is of vegetable origin is no longer questioned. Such 

 a mode of origin is directly proven by microscopical examination, 

 which shows that even the hardest anthracite is a mass of car- 

 bonized but determinable vegetable fibres. On the other hand, 

 there has been much difference of opinion concerning the way in 

 which such immense masses of vegetable matter were brought 

 together. Much the most probable view is, that the coal was 

 formed in position in great peat bogs, added to, no doubt, by 

 more or less drifted material. The evidence for this view is to 

 be found: (1) in the great extent and uniform thickness and 

 purity of many coal seams, which we cannot account for in any 

 other way. Had the vegetable matter been largely drifted 

 together, it must have been contaminated with sediment and 

 could not have been spread out so evenly over great areas. 

 This objection to the "driftwood theory" becomes all the 



