THE CARBONIFEROUS PERIODS 503 



sents only about five per cent, of the original thickness of the peat 

 bed, consequently a coal bed 16 feet thick represents a peat bed 

 about 320 feet in thickness. None of the peat bogs of the present 

 have the great thickness of peat necessary to make great beds of 

 coal. 



The commercial importance of Great Britain and much of the re- 

 markable development of the United States are due to the presence 

 of abundant and accessible supplies of coal. 



Climate during the Deposition of Coal. — Since coal occurs not 

 only in the temperate zones and in the tropics but even in the polar 

 regions, it has been assumed that the climate of the Pennsylvanian 

 was uniform throughout the world. This is further borne out by 

 a study of the structure of the wood, which shows no rings of growth 

 such as are developed in -plants living in a climate in which there 

 are dry and wet or cold and warm seasons. The question as to the 

 temperature and the amount of moisture has given rise to some dis- 

 cussion. It has been generally assumed that the large size of many of 

 the trees and the accumulation of their remains in swamps are proofs 

 of a warm, humid climate. The thickness of the coal seams has also 

 been considered confirmatory evidence. Several objections have 

 been offered to this belief, however. (1) At present the great accu- 

 mulations of peat are in cold, temperate climates. (2) Peat is rarely 

 formed of rapid-growing plants but chiefly of the remains of such 

 plants as sphagnum moss. (3) It has been pointed out that, as a 

 whole, the leaves of Carboniferous plants bear a resemblance to those 

 of living plants that are adapted to dry (xerophytic) conditions (p. 

 497), being narrow and possessing devices to prevent the rapid evapo- 

 ration of water. In the tropics peat accumulates more from tree 

 trunks and leaves which have been floated into lakes and marshes, and 

 little from moss or trees that grew in situ. 



When all the evidence is considered, there seems little doubt that 

 the climate of the regions in which coal accumulated was moist and 

 warm, although not tropical. 



REFERENCES FOR COAL 



Jeffrey, E. C, — The Mode of Origin of Coal: Jour. Geol., Vol. 23, pp. 213-230. 

 Savage, T. E., — On the Conditions under which the Vegetable Matter of the ltKnois 



Coal Beds Accumulated : Jour. Geol., Vol. 22, pp. 750-765. 

 Stevenson, J. J., — Formation of Coal Beds. 

 Tonge, James, — Coal. 



