512 PAI^EONTOLOGICAL REPORT OF GEOLOGICAL SURVEY. 



is often found somewhere reposing on their surface. The fire-clay is 

 generally a reliable guide for the identification of veins, when it sepa- 

 rates two beds of coal, forming what is generally called a clay parting. 

 In this case, it is ordinarily found, though of variable thickness, over 

 a wide extent. But it is then formed like the shales; in some cases, it 

 is even a true shale, and it is in the examination of the shales that 

 the reason of its formation, and of its appearance, ought to be looked 

 for. 



2nd. THE COAL? 



There is no substance of which so many chemical analyses have 

 been made, and none, also, of which the chemical elements are so well 

 known. The general result of all these analyses has proved a curious 

 fact, viz : that two pieces of coal, taken from the same bed, at only a 

 few feet distance, have scarcely ever presented exactly the same propor- 

 tions in the quantity of their essential compounds. The reason of this 

 is easily understood: each plant, especially each kind of tree, has for its 

 wood a peculiar composition; each one is more or less resinous, hard or 

 porous, has more or less of woody matter in an equal volume, and each 

 plant has a peculiar acid ; all the essential elements are locally preserved 

 in the coal. The same remark is true of beds of peat, of which two slices 

 cut either horizontally or vertically, at a distance of one or two feet from 

 each other, never present exactly the same appearance, nor exhibit ex- 

 actly the same proportion in their chemical elements. Some plants of the 

 coal — the Catamites and the Stigmaria especially, fix in their tissue the 

 silica of the water, and the quantity of ash varies in proportion to their 

 abundance in the coal. Some others are porous, and when lying on the 

 surface of ;i bed of coal, they let particles of mud percolate through, or 

 within their tissue, and produce the same result in another way, and at 

 another place. From these different causes, the ashes of the coal have a 

 different color, and the distinction of white ash and red ash coal, which 

 may be of great moment in the identification of the beds of part of a 

 basin, is, when considered in a general point of view, of little value. If 

 we may rely on the sections of the anthracite basins of Pennsylvania 

 as they are generally given, the upper beds of it belong to the red, the 

 intermediate ones to the grey, and the lower ones to the white ash se- 

 ries. In the coal-fields of western Kentucky and of Illinois, the 

 upper beds of coal are white ash, the middle ,qnes red, and the lower 



