THE HISTORY OF PLANT LIFE. 25 



of the Ohio shales, which are remarkable in some localities for 

 the vast number of spores of Protosalvinia, associated with oil, 

 and in such a state of preservation that they may be recognized 

 very readily. The extreme form of such alteration is to be 

 found in graphite, while the various forms of anthracite, bitu- 

 minous coal, lignite and peat indicate in inverse order the 

 successive stages through which the plant remains pass. It 

 follows from these considerations that, as a rule, we can expect 

 little structure to be exhibited in coaly masses, but we may 

 draw correct conclusions as to the general character of the 

 parts represented by taking into consideration the position in 

 the specimen and the original nature of the various tissues. 

 Thus we know that unmodified cellulose, such as constitutes the 

 soft parts of plants generally, contains carbon 44.55^), hydro- 

 gen 6. 14^, and oxygen 49.51^. Lignin, or the essential basis 

 of all woody structures, contains C. 62.25^, H. 5.93^, O. 

 36.82^, while cutin and suberin, the basis of cuticle, the walls 

 of spores and of cork, contain C. 73~74/&, H. io$fo, O. 16-17^). 

 A careful consideration of these figures leads to most important 

 and interesting conclusions, since the durability of parts, or 

 their ability to resist decay, as also the readiness with which 

 they pass into the form of carbonized remains, is in direct pro- 

 portion to the relative excess of carbon in the original structure. 

 Hence it is not difficult to understand why the more perishable 

 fundamental tissues of plants are so rarely preserved while the 

 woody and cortical parts are well preserved, and why the latter 

 particularly may remain as a shell of coal when all other parts 

 have been removed by decay or replaced by mineral matter. 

 In the light of these facts, also, the remarkable preservation of 

 many spores may be readily understood. 



As carbonization proceeds, the material may become satu- 

 rated with water holding in solution small quantities of carbonate 

 of lime or of silica, in which case the entire structure becomes 

 gradually converted into a mass of calcite or of silica, as the case 

 may be, the displacement of the organic matter proceeding at 

 so gradual a rate that all the structural features are retained. 

 The structure is then represented by fine particles of carbon 

 disposed along the original lines of structure, while the 



