44 CARBONIGENOUS ERA. 



probable, from similar vegetation being now found in such 

 situations within the tropics. 



With regard to the circumstances under which the masses 

 of vegetable matter were transformed into successive coal 

 strata, geologists are divided. From examples seen at the 

 present day, at the mouths of such rivers as the Mississippi, 

 which traverse extensive sylvan regions, and from other cir- 

 cumstances to be adverted to, it is held likely by some that 

 the vegetable matter, the rubbish of decayed forests, was 

 carried by rivers into estuaries, and there accumulated in vast 

 natural rafts, until it sunk to the bottom, where an overlay of 

 sand or mud would prepare it for becoming a stratum of coal. 

 Others conceive that the vegetation first passed into the con- 

 dition of a peat moss, that a subsidence then exposed it to be 

 overrun by the sea, and covered with a layer of sand or mud ; 

 that a subsequent uprise made the mud dry land, and fitted 

 it to bear a new forest, w*hich afterwards, like its predecessors, 

 became a bed of peat ; that, in short, by repetitions of this 

 process, the alternate layers of coal, sandstone, and shale, con- 

 stituting the carboniferous group, were formed. It is favour- 

 able to this last view that marine fossils are rarely found in 

 the body of the coal itself, though abundant in the shale layers 

 above and below it ; also that in several places erect stems of 

 trees are found with their roots still fixed in the shale beds, 

 and crossing the sandstone beds at almost right angles, show- 

 ing that these, at least, had not been drifted from their ori- 

 ginal situations. On the other hand, it is not easy to admit 

 such repeated risings and sinkings of surface as would be 

 required, on this hypothesis, to form a series of coal strata. 

 Perhaps we may most safely rest at present with the suppo- 

 sition that coal has been formed under both classes of circum- 

 stances, though in the latter only as an exception to the 

 former. 



The plants of the carbonigenous period have been investi- 

 gated with great care by several able naturalists, and above 

 eight hundred species have been ascertained. The living plants 

 of our own era are at least 80,000, and it is difficult to suppose 

 the flora of that remote age to have been so much more 



