360 SCIENCE PROGRESS. 



the growth-in-place method of formation he speaks of as the 

 autochthonous origin of coal, and the building up of coal 

 from drifted material is designated the allochthonous mode 

 of origin. 



Among the older views as to the nature and origin of 

 coal, we occasionally meet with the suggestion that the 

 greater part of the carbonaceous substance has been derived 

 from bituminous eruptions. It has been generally agreed 

 that coal is almost entirely made up of carbonised plant 

 fragments, and the idea of any extraneous source of carbon 

 has been allowed to drop. Recently, however, this old 

 theory has been revived, and some new arguments set for- 

 ward in its support. Before passing on to consider the 

 allochthonous mode of formation, we must take note of this 

 third theory which M. Rigaud has seriously discussed in a 

 recent number of the Revue Scicntifique} This writer's 

 main contention is that plants have played a subordinate 

 role in the formation of coal, and are by no means mainly 

 responsible for its production. 



Coal usually exhibits traces of plant tissues embedded in 

 a black substance, and this homogeneous matrix may be 

 regarded either as a bituminous substance of volcanic origin 

 or as a product of vegetable decomposition. Assuming a 

 tropical climate for Coal-Measure times, Rigaud points out 

 the unsuitable nature of tropical plants, and the still more 

 unfavourable climatal conditions for the formation of any- 

 thing of the nature of peat. 



Neumayr and many other writers have drawn attention 

 to the absence of peat in tropical countries, and have used 

 this fact either as an argument against a tropical climate 

 during the coal period, or as an obstacle to the growth-in- 

 place method of accumulation. 



Rigaud lays stress on the absence of certain elements in 

 the ash of coals, which ought to be present, on the assump- 

 tion that the carbon has been derived from plant tissues. If 

 coal consists of altered vegetable dtti'is, we ought to find 

 a certain amount of alkalies and phosphoric acid in its ash ; 



1 I am indebted to Prof. Zeiller for calling my attention to this article. 



