§ 1. INTRODUCTORY. 66L 



The Carboniferous system, so named from its comprising 

 the principal deposits of mineral fuel, consists of sand- 

 stones, bituminous shales, bluish grey limestones, clays with 

 iron ore, and quartzose grits; between which are inter- 

 calated seams and thick beds of the carbonized vegetable 

 matter, termed Coal. Independently of the interest 

 attached to these deposits from the immense accumu- 

 lation of fossil plants of which many of them are wholly 

 composed, this system involves the consideration of 

 some very remarkable geological phenomena; for the 

 manner in which such extensive layers of carbonized vege- 

 table substances, unmixed with extraneous matter, were 

 produced, is a problem difficult of solution, under cer- 

 tain conditions in which it is presented to our exami- 

 nation. Like the intercalation of the masses of rock-salt 

 in the red marls of the Trias (ante, p. 539), the deposition 

 of coal appears, as we shall presently show, to have taken 

 place under various circumstances ; the coal being in some 

 cases associated with fresh- water, and in others with marine 

 organic remains. 



But though from the vast importance of mineral fuel in 

 an economical point of view to nations in an advanced state 

 of civilization, and the botanical interest with which such 

 extensive natural herbai'ia of the palseozoic ages are in- 

 vested, the coal is generally regarded as constituting the 

 essential feature of this epoch, yet it would be more 

 philosophical to consider these intercalations of carbonized 

 vegetables, as extraneous and accidental. We have al- 

 ready seen, that the formation of coal was not confined to 

 the carboniferous system ; but that beds of this substance 

 have been and will be produced, wherever trees and plants 

 are accumulated in sufficient quantity, and under the re- 

 quisite conditions. 



The peculiar types of vegetable organization comprised 

 in the flora of this system, afford the only distinguishing 

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