THE CARBONIFEROUS PERIODS 505 



draining of the swamps, with the resulting death of the swamp vege- 

 tation, or to the spreading of upland trees which existed in the Penn- 

 sylvanian but of which nothing is now known, cannot be stated. 

 Once well-established, however, the more highly organized upland 

 plants probably became in time suited to swamp conditions and 

 occupied the places formerly held by the lycopods and horsetails. 



(2) Why was the Permian a period of glaciation, and in particular, 

 why were the areas affected not only near the equator but near sea 

 level ? 



Various explanations have been offered, but none has a general acceptance. One 

 is based on the assumption that the carbon dioxide contents of the air was decreased. 

 The depletion of carbon dioxide is believed, by the adherents of this theory, to ha^e 

 been the result of a combination of causes, indirectly because of the elevation of the 

 continents and an increase in the land area. As a result of a greater land surface being 

 exposed to the agents of the weather, the rocks upon weathering extractejd_mudi_car- 

 bon dioxide from the air. The depletion of this gas was further hastened by the oceans, 

 which are believed by the supporters of this theory to have absorbed great quantities 

 of carbon dioxide. The air was also freed of its carbon dioxide during the accumula- 

 tion of coal. One doubtful element in the theory is the efficacy of carbon dioxide in 

 retaining heat. 



Another solution of the problem is found in the amount of water vapor in the at- 

 mosphere, since water vapor is known to act as a blanket in retaining heat. The en- 

 larging of the land area decreased the amount of water vapor in the atmosphere and 

 thinned the thermal blanket. 



(3) Why was the climate generally arid in the northern hemisphere 

 during the Lower Permian ? The great extent of land and the nar- 

 rowing of the oceans undoubtedly had a marked effect in producing 

 an arid climate, since less moisture would have been evaporated and it 

 would have been precipitated over a wider area. 



(4) Why was the great Appalachian system raised at this time ? 

 It is usually stated that strains had been accumulating in the earth's 

 crust throughout the Paleozoic and that these strains were relieved 

 by the folding of the Appalachian trough at its close, but this does not 

 fully answer the question. 



Summary of the Paleozoic Era 



The Building of the Continents. — The continent of North America 

 was probably covered by seas in every part during some portion of the 

 Paleozoic, but two areas seem to have been especially free from epi- 

 continental seas during all but perhaps a small part of the era. These 

 areas lie in the Laurentian region of eastern Canada and in the sourh- 



