INTRODUCTION 9 



Among internal casts the most important examples 

 are those which filled the medullary cavity of the 

 Calamites or arboreal Horsetails ; these casts, repro- 

 ducing exactly the internal surface of the woody 

 cylinder, constitute the most familiar remains of this 

 group of plants, and were long confused with the stems 

 themselves. 



Seams of coal have been regarded as a case of 

 incrustation on a great scale. 1 Here it is not single 

 specimens, but whole masses of -vegetable remains which 

 have been incrusted by mineral matter. The remains 

 themselves have been converted into the more or less 

 structureless carbonaceous material which we call coal, 

 but on the upper and lower shales we find impressions 

 of the plants which happen to have been in contact 

 with the adjacent mineral deposits. The coal itself 

 has undergone great chemical changes, the organic 

 substance having been almost entirely converted into 

 various hydrocarbons. In certain cases, however, 

 sufficient structure has been preserved to enable us to 

 form some idea of the kinds of plants to which the 

 coal owed its origin. 



We now come to the other great mode of preserva- 

 tion, petrifaction, in which the whole of the organised 

 substance has been completely saturated by mineral 

 matter in solution, which has subsequently been pre- 

 cipitated in a solid form. In this case alone is the 

 structure thoroughly preserved. There have been 

 many petrifying agents ; the two most important for 

 us are silicic acid (H 4 Si0 4 ), and calcium carbonate 

 (CaCO ). The most interesting vegetable remains 



1 Solms-Laubach, I.e. chap. i. 



