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 2 sz¢«: and as the plants of the 
coal are not marine 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 coloyr and texture. All the 
varieties of coal, also, consist chemically of cardon, with vary- 
ing proportions of certain gaseous constituents and a small 
amount of incombustible mineral or ‘fash.” 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 Lepzdodendra). 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 
