312 HISTORICAL GEOLOGY. 



Physical Geography and Climate of the Coal Period. 



Physical Geography. The Paleozoic era was a time 

 of gradual growth of the continent from the Archaean 

 nucleus by successive additions, first of the Silurian., then 

 of the Devonian, and now of the Carboniferous area. 

 During Carboniferous times the form of the American 

 Continent probably did not differ greatly from that repre- 

 sented on page 349 (Fig. 303) as the Cretaceous conti- 

 nent, except that the areas of coal-measures were not 

 then permanent land, but were in an uncertain state, 

 sometimes swamp-land, sometimes covered with river- 

 sediment, sometimes covered by the sea. Although the 

 continent had greatly grown, still we must imagine it 

 as small and low compared with its present state. The 

 same is probably true of other continents. 



Climate. The climate was probably warm, very moist, 

 very uniform, and the air loaded with CO^. The greater 

 warmth and uniformity are shown by the fact that the 

 plants are those of a tropical climate. Tree-ferns, arbo- 

 rescent lycopods, etc., grew then with ultra-tropical lux- 

 uriance, not only in now temperate regions, but in Mel- 

 ville Island and Grinnell Land, 78-80 north latitude. 

 The prevalence of the great coal-swamps and the charac- 

 ter of the plants are sufficient evidence of greater humid- 

 ity. Finally, when we remember that the whole of the 

 coal in the world represents so much carbon taken from 

 the 'atmosphere, as C0 2 with return of the oxygen, we 

 shall be convinced that the quantity of C0 a in the air 

 was greater and of oxygen less than now. 



It is probable, therefore, that in early geological times 

 there were more moisture and C0 2 and less oxygen than 

 now. This would make a paradise for plants, especially 

 the lower orders, but would be unsuitable for air-breath- 

 ing animals. There has been throughout all geological 



