44 ERA OF THE CARBONIFEROUS FORMATION". 



have a comparatively brief period of volcanic disturbance 

 (when the conglomerate was formed.) Then the causes 

 favorable to the so abundant production of limestone, and 

 the large population of marine acrita, decline, and we find 

 the masses of dry land increase in number and extent, and 

 begin to bear an amount of forest vegetation, far exceed- 

 ing that of the most sheltered tropical spots of the present 

 surface. The climate, even in the latitude of Baffin's Bay, 

 was torrid, and perhaps the atmosphere contained a lar- 

 ger charge of carbonic acid gas (the material of vegetation) 

 than it now does. The forests or thickets of the period, 

 included no species of plants now known upon earth. 

 They mainly consisted of gigantic shrubs, which are either 

 not represented by any existing types, or are akin to kinds 

 which are now only found in small and lowly forms. 

 That these forests grew upon a Polynesia, or multitude of 

 small islands, is considered probable, from similar vege- 

 tation being now found in such situations within the tro- 

 pics. With regard to the circumstances under which 

 the masses of vegetable matter were transformed into suc- 

 cessive coal strata, geologists are divided. From exam- 

 ples seen at the present day, at the mouths of such rivers 

 as the Mississippi, which traverse extensive sylvan re- 

 gions, and from other circumstances to be adverted to, it 

 is held likely by some that the vegetable matter, the rub- 

 bish of decayed forests, was carried by rivers into estua- 

 ries, and there accumulated in vast natural rafts, until it 

 sunk to the bottom where an overlayer of sand or mud 

 would prepare it for becoming a stratum of coaL Others 

 conceive that the vegetation first went into the condition 

 of a peat-moss, that a sink in the level 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, which afterwards, like its 

 predecessor, became a bed of peat ; that, in short, by repe- 

 titions of this process, the alternate layers of coal, sand- 

 stone, and shale, constituting the carboniferous group, were 

 formed. It is favorable to this last view that marine fos- 

 sils are scarcely 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 sand- 

 itone beds at almost right angles, showing that these, at 

 least, had not been drifted from their original situations 



