Page:A Treatise on Geology, volume 1.djvu/187

 CHAP. VI.



Of the accumulated remains of these plants coal seams are really composed, and one cause of the differences amongst them is the different structural composition of the original plants. How far the above fossil flora is to be taken as exhibiting the true proportions of the tribes of plants living on the globe, at the time of the production of the rocks of the carboniferous system, is uncertain: since, when plants are swept down from the land into the sea, it depends on many unknown conditions what sorts of them shall escape the floods, or perish by maceration in the waters.

As a general rule, it may be said that the plants are confined to arenaceous and argillaceous deposits: they