AS COMPARED WITH THAT OF THE PRESENT DAS'. 389 



t 



stone. Had it not been for these favourable circumstances, the study of 

 coal fossils would have been apparently hopeless ; for, whilst the clays and 

 ironstone and sandstone scarcely ever contain more than one large class 

 (ferns) in a fit state for determination, the shales preserve only the out- 

 lines of another {Sigillarice and Lepidodendrons) , whose affinities could 

 hardly be guessed without a microscopical examination of their internal 

 tissues, as these are preserved in the ironstones and sandstones. Con- 

 sidering of how exceedingly lax and compressible a tissue the coal- 

 plants were composed, it is not wonderful that instructive specimens 

 are rare ; but to appreciate to its full extent how universal is the com- 

 pression and how complete the mutilation of almost every individual, it 

 is necessary to study the whole bed or deposit, in situ. Thus will be 

 seen a layer of mineralized organic matter, exceeding in bulk and in 

 area whatever any other formation may present in equal purity ; for 

 throughout the whole mass of the coal there will not be found one 

 pebble or even grain of sand : it is a deposit of vegetable matter, so 

 homogeneous that not a trace remains of the outward form of that 

 incalculable number of species and specimens of plants to whose decay 

 it owes its existence. 



Plants, whose tissues are so lax as to be convertible after death 

 into a mass of such uniform structure as coal, evidently would not 

 retain their characters well during fossilization, under whatever favour- 

 able circumstances that operation may be conducted. We conse- 

 quently find that few specimens are available for scientific purposes. 

 Of the ferns, whose remains preponderate in the carboniferous Flora, 

 only one surface of the leaves or fronds, and that invariably the least 

 important (botanically speaking), is exposed to view ; and their mutila- 

 tion is so great that the identification of contiguous specimens is fre- 

 quently impossible, much more so of those from different parts of the same 

 or from other coal-fields. Were the species and genera identical with 

 those now in existence, this difficulty would be lessened, for we should 

 then know the variations in form which the individuals might be likely 

 to assume, or, at any rate, what dissimilarity between the isolated frag- 

 ments was due to their belonging to different parts of the same plant. 

 The naturalist is thus hampered in the outset by his inability to answer 

 questions relating to systematic and specific botany. And when he 

 turns to a general review of the whole, and seeks to reclothe the globe 

 with the vegetation to whose decomposition we are indebted for coal, 

 he labours under no lighter difficulties : the most casual inspection of 

 such a wreck suffices to show that the number of species, genera, and 

 even orders, of which scarce a trace remains, must far outnumber 

 those which are recognizable. Of the latter, again, but a small per- 

 centage is known in a tolerably complete state, only the larger and 



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