AS COMPARED WITH THAT OP THE PRESENT DAY. 393 



t 



ation of certain areas to produce those vast accumulations, the expla- 

 nation of whose origin is still an unsolved problem. Other questions, 

 which a study of living plants alone can answer, refer to the sorts of 

 plants best calculated to thrive in such a uniform soil as the under- 

 clay, upon which each bed of coal rests, and into which some of the 

 vegetables have certainly been rooted. What form of surface is best 

 fitted to retain so mobile a debris as the coal was previous to its 

 compression and hardening; what degree of dryness would be most 

 favourable to such an accumulation, consistently with an energetic 

 growth of vegetation. 



The above considerations pre-suppose some general ideas of the 

 vegetations both of the tropics and cooler latitudes, of mountain-chains, 

 table-lands, valleys, and actuaries, more especially of countries cha- 

 racterized by equable or by excessive or extreme climates, as compared 

 with continents, and of humid and desert districts ; in short, of all the 

 complex associations with, or dependence of botanical characters upon, 

 surface, soil, and climate, which the globe presents. 



The want of this kind of information amongst many naturalists, and 

 the neglect of its application by others, have caused those utterly con- 

 tradictory opinions which have been expressed regarding the origin of 

 coal,* and unnecessarily complicated the subject. The botanist must 

 not seek to force a plant into a natural order, the habits of whose ex- 

 isting species are incompatible with those conditions under which a more 

 comprehensive view of the coal formation may assure him it must have 

 vegetated ; nor can the geologist put forward any theory which will 

 explain the features of that formation, if it be grounded on views op- 

 posed to those few certain data, which a study of the botany of the 

 period in question has afforded. 



There is another branch of this investigation of equal importance to 

 the geologist and botanist, namely, the identification and comparison of 

 the species from different and sometimes remote coal-fields, or from the 

 various strata of the same field. This is as difficult as any of the 

 points which occupy the botanist ; and all questions connected with the 

 geographical distribution of the plants of that period being dependent 

 on the results thus obtained, it is one which requires extreme caution 

 in the working. The obvious tendency in the student is to regard as 



* The looseness of the speculations hitherto advanced on the relationship of the coal flora 

 to such physical conditions as climate, cannot he better illustrated than by the fact that the 

 Sigillarice (which have undoubtedly contributed largely to the formation of coal) are 

 considered by some naturalists to be allied to the order Euphorbiacece, by others to Cacti, 

 and by the majority to ferns. The necessary conclusion to which those who place them 

 in the first two orders would lead us, is, that they were inhabitants of singularly arid and 

 desert countries ; whilst, if ferns, they are characteristic of diametrically opposite condi- 

 tions, a moist soil and a humid atmosphere. 



