THE CARBONIFEROUS PERIOD. 163 



and deposited directly on the bottom of the sea ; but in the 

 majority of cases the coal is undeniably the result of the slow 

 growth and decay of plants in situ : and as the plants of the 

 coal are not mariiie plants, it is necessary to adopt some such 

 theory as the above to account for the formation of coal- 

 seams. By this theory, as is obvious, we are compelled to 

 suppose that the vast alluvial and marshy flats upon which the 

 coal-plants grew were liable to constantly-recurring oscillations 

 of level, the successive land-surfaces represented by the suc- 

 cessive coal - beds of any coal - field being thus successively 

 buried beneath accumulations of mud or sand. We have no 

 need, however, to suppose that these oscillations affected large 

 areas at the same time; and geology teaches us that local 

 elevations and depressions of the land have been matters of 

 constant occurrence throughout the whole of past time. 



All the varieties of coal (bituminous coal, anthracite, cannel- 

 coal, &c.) show a more or less distinct "lamination" — that is 

 to say, they are more or less obviously composed of successive 

 thin layers, differing slightly in colour and texture. All the 

 varieties of coal, also, consist chemically of caj^bon, with vary- 

 ing proportions of certain gaseous constituents and a small 

 amount of incombustible mineral or " ash." By cutting thin 

 and transparent slices of coal, we are further enabled, by 

 means of the microscope, to ascertain precisely not only that 

 the carbon of the coal is derived from vegetables, but also, in 

 many cases, what kinds of plants, and what parts of these, enter 

 into the formation of coal. When examined in this way, all 

 coals are found to consist more or less entirely of vegetable 

 matter ; but there is considerable difference in different coals as 

 to the exact nature of this. By Professor Huxley it has been 

 shown that many of the English coals consist largely of ac- 

 cumulations of rounded discoidal sacs or bags, which are 

 unquestionably the seed-vessels or " spore-cases " of certain of 

 the commoner coal-plants (such as the Lepidodendrd). The 

 best bituminous coals seem to be most largely composed of 

 these spore-cases ; whilst inferior kinds possess a progressively 

 increasing amount of the dull carbonaceous substance which is 

 known as "mineral charcoal," and which is undoubtedly com- 

 posed of '' the stems and leaves of plants reduced to little 

 more than their carbon." On the other hand, Principal Daw- 

 son finds that the American coals only occasionally exhibit 

 spore-cases to any extent, but consist principally of the cells, 

 vessels, and fibres of the bark, integumentary coverings, and 

 woody portions of the Carboniferous plants. 



The number of plants already known to have existed during 



