COAL: ITS STRUCTURE AND FORMATION. 359 



too complex to allow us to assign definite reasons for the 

 present form of carbonaceous deposits. The element of 

 time is constantly referred to as one of the guiding factors 

 in the formation of lignitic deposits and true coals, but 

 granting- its importance in geological changes, it has, in 

 all probability, been drawn upon too freely as a means of 

 accounting for certain phenomena. The evidence of recent 

 research seems to point very distinctly to a much more 

 rapid formation of coal than has previously been supposed. 

 It would seem that we have no good grounds for asserting 

 that modern peat formations, or certain Mesozoic and Ter- 

 tiary lignites, would ever assume the characters of true coals, 

 however much time be allowed for future changes. There 

 has been at certain times in the earth's history a concur- 

 rence of special conditions, which have rendered possible the 

 formation of coal deposits on a large scale, and these con- 

 ditions were especially characteristic of Upper Carboniferous 

 times. We are certainly not justified in adopting Lesquereux' 

 dictum 1 that peat bogs are nothing but beds of coal " not 

 entirely ripe or burned out ". The central idea of the 

 growth-in-place theory may be summed up in a sentence 

 from Geikie's text-book of geology : " Each coal seam re- 

 presents the accumulated growth of a period which was 

 limited either by exhaustion of soil, or by the rate of the 

 intermittent subsidence that affected the whole area of coal- 

 growth ".'- 



Following the early views of Sternberg and others, 

 several writers have in recent years advocated in some 

 form or other the formation of coal strata by the drifting of 

 vegetable ddbris, and its subsequent deposition on the floor 

 of a lagoon or sea, with an accompanying series of 

 arenaceous and argillaceous sediments. 



Theories of this class which do not regard coal seams as 

 old peat bogs, or as the remains of forests which grew on 

 the underclay soil, are usually referred to collectively as the 

 drift theory of coal formation. 



For these opposing sets of views Gumbel 3 has instituted 

 two new terms which have been adopted by a few writers ; 



1 Lesquereux, p. 842. - Geikie, p. 808. 3 Gumbel, p. 201. 



