COAL: ITS STRUCTURE AND FORMATION. 439 



varieties and carbonaceous shale to ordinary shale or grit ; 

 and, indeed, the stratigraphical evidence unmistakably 

 suggests that we must look upon beds of coal as units in a 

 series of strata built up on an area of deposition according 

 to a common law of sedimentation. The chemistry of coal 

 formation is too wide a theme to deal with in an article 

 which has already extended beyond reasonable limits. The 

 processes of change seem to have gone on much more 

 rapidly than is usually supposed, if we endorse the conclu- 

 sions of Renault and others. Some writers consider the coaly 

 transformation of plant tissues must have required not only 

 pressure, but a considerable amount of heat. Others, 

 again, believe that a mass of decaying vegetable matter 

 has undergone a kind of peaty fermentation, resulting in 

 the partial carbonisation of the tissues, and the production 

 of various ulmic substances. This vegetable sediment 

 would be spread out on the floor of a lagoon, and eventually 

 covered with mud or sand, when the further stages in the 

 process of carbonisation would be completed by a gradual 

 desiccation of the whole mass under some pressure and a 

 comparatively low temperature. 



Much has been written as to the reduction in bulk of 

 vegetable tissues as a result of their conversion into coal. 

 The late Dr. Stur attempted to estimate the amount of 

 decrease in volume, and suggested a simple formula by 

 which to calculate the original diameter of a plant's stem or 

 root, from the thickness of the coaly layer which frequently 

 envelops a pith cast in sandstone or shale. Calculations 

 have been made by other palaeobotanists, which differ con- 

 siderably from those of Stur, but as Potonie 1 safely suggests, 

 we cannot expect to arrive at any numerical estimate of the 

 volume reduction which may be applied to all cases. 



Neglecting for the present the probable climatic condi- 

 tions which characterised the coal period forests, and the 

 discussion of the biology of coal-forming plants, we may, in 

 conclusion, make brief reference to a few of the numerous 

 descriptions of recent geographical conditions which have 



1 Potonie (2). 



