414 THE CARBONIFEROUS PERIOD 



stronger when it is remembered that the process of converting 

 vegetable matter into coal greatly reduces its bulk, a given thick- 

 ness of coal representing only about 7% of the original thickness 

 of vegetable substance. Thus a 20- foot seam of coal implies 

 the accumulation of nearly 300 feet of plants, and it is highly 

 improbable that such a mass could have been evenly spread as 

 drift over hundreds (or even thousands) of square miles, without 

 a large admixture of mud or sand. (2) The very general presence 

 of the underclay beneath coal seams points to the same conclusion. 

 An underclay, as we have seen, is an ancient soil, and is of just 

 the same character as that which we find under such modern 

 peat bogs as the Great Dismal Swamp (see pp. 134-135). 



The subsidence of the bogs and the deposition of sediments 

 upon them gradually built up the great series of strata which are 

 called the coal measures. The peat was thus subjected to the 

 steadily increasing pressure of the overlying masses, which greatly 

 aided in the transformation of the vegetable accumulations into 

 coal. Where the coal measures have been folded, the still greater 

 pressure, aided by heat, and perhaps by steam, has resulted in the 

 formation of anthracite. The greater number of the Carboniferous 

 bogs appear to have been covered by fresh water, though some 

 were coast swamps, extending out into brackish or even salt water. 



The workable coal fields of North America, belonging to the 

 Carboniferous system, are found in several distinct areas, some 

 of which were doubtless separate basins of accumulation, while 

 others have become disconnected by denudation. 



(1) In the xAcadian province the coal measures occur in the 

 island of Cape Breton, Nova Scotia, and New Brunswick ; in Nova 

 Scotia they are of immense thickness, 7000 feet, with 6000 feet 

 of underlying conglomerate. A second basin of this province is 

 near Worcester (Mass.), and a third extends through Rhode Island 

 into southeastern Massachusetts. The latter basins are metamor- 

 phic and yield a very hard anthracite. 



(2) The great Appalachian field has an area of more than 

 50,000 square miles. It covers most of central and western 

 Pennsylvania, eastern Ohio, western Maryland and Virginia, and 



