THE GROWTH OF COAL 249 



dantly on the higher levels. Nor was strictly aquatic vegetation 

 wanting. We find, both in the preceding Devonian and the 

 Carboniferous, that the little aquatic plants now known as 

 Rhizocarps, and structurally allied to the Ferns such plants 

 as the floating Salvinia, and the Pillworts of our swamps, were 

 vastly abundant, and they may have filled and choked up with 

 their exuberant growth many of the lakes and slow streams of 

 the period, furnishing layers of cannel and " macrospore " 

 coal, and earthly bitumen or Torbanite. 



We have hitherto confined our attention to the great Car- 

 boniferous period, so called, as emphatically the age of coal ; 

 but this mineral, and allied forms of carbon, were produced 

 both before and after. Even in that old Laurentian age, 

 which includes the oldest rocks that we know, formed when 

 the first land had just risen out of the waters, there are thick 

 beds of graphite, or plumbago, chemically the same with 

 anthracite coal, and which must have been produced by the 

 agency of plants, whether terrestrial or aquatic. We may sup- 

 pose that the plants of this remote age were of very humble 

 type as much lower than those of the coal formation as these 

 are lower than those of the present day ; but if so, then, on the 

 analogy of the Carboniferous, they would be high and complex 

 representatives of those low types. But there is another and 

 more startling possibility ; that the Laurentian may have been 

 a period when vegetable life culminated on the earth, and 

 existed in its most complete and grandest forms in advance of 

 the time when it was brought into subordination to the higher 

 life of the animal. In the meantime, the Laurentian rocks are 

 in a state of so extreme metamorphism that they have afforded 

 no certain indication of the forms or structures of the vegeta- 

 tion of the period. 



We find indications of plant life through all the Paleozoic 

 groups succeeding the Laurentian ; but it is not till we reach 

 the Devonian, the system immediately preceding the Carboni- 



