68 PAL^ONTOLOGICAL BOTANY. 



boniferous deposits accumulated were low, is proved not only 

 by the occasional association of marine remains, but by the 

 enomious thickness of strata of shale and sandstone to which the 

 seams of coal are subordinate. The coal-measures are often 

 thousands of feet, and sometimes two or three miles, in yerti- 

 cal thickness, and they imply that for an indefinite number of 

 ages a great body of water flowed continuously in one direc- 

 tion, carrying doTvn towards a given area the detritus of a 

 large hydrographical basin, draining some large islands or 

 continents, on the margins of which the forests of the coal 

 period grew. If this view be correct, we can know little or 

 nothing of the upland flora of the same era, still less of the 

 contemporaneous plants of the mountainous or alpine regions. 

 If so, this fact may go far to account for the apparent mono- 

 tony of the vegetation, although its uniform character may 

 doubtless be in part oAving to a greater uniformity of climate 

 then prevailing throughout the globe. Mr. Bunbury has suc- 

 cessfully pointed out that the peculiarity of the carboniferous 

 climate consisted more in the humidity of the atmosphere and 

 the absence of cold, or rather the equable temperature pre- 

 served in the different seasons of the year, than in its tropical 

 heat ; but we must still presume that colder climates existed 

 at higher elevations above the sea." 



The plants of the coal-measures are evidently terrestrial 

 plants. Brongniart agrees with Lyell in thinking that the 

 layers of coal have in general accumulated in the situation 

 where the plants forming them grew. The remains of these 

 plants covered the soil in the same way as layers of peat, or 

 the vegetable mould of great forests. In a few instances, 

 however, the plants may have been transported from a dis- 

 tance, and drifted into basins. Phillips is disposed to think 

 that this was the general mode of formation of coal-basins. 

 He is led to this conclusion by observing the fragmentary 

 state of the stems and branches, the general absence of roots, 



