﻿CHAPTER II. 



PRESERVATION AND EXTERNAL CHARACTERS. 



VARYING CONDITIONS OF FOSSILIZATION AND RESULTING TRUNK FORMS. 



The mineralization of entire plants as fossils is conditional, firstly, upon the 

 tissue systems present, as connected with particular stages of growth, prefoliation, 

 prerloration, or fructification. The secondary factors of control in the process are 

 mainly : (i) the relative abundance and kind of mineralizing materials ; (2) tem- 

 perature ; (3) the presence or absence of secondary reagents, such as iron, capable of 

 replacing plant tissues and preserving their microscopic structures in finely differ- 

 entiated form, but not necessary to the process of silicification or calcification, as the ' 

 case may be ; (4) the duration and rapidity of chemical activity ; (5) the nature of 

 the embedding rock material. These are the principal elements determining the 

 clearness with which structural details are preserved and differentiated, the final 

 results being in addition dependent upon the freedom from maceration or decay of 

 the original plants at the time of their fossilizatiou, as well as from subsequent 

 chemical changes or compression in the containing beds after the early process of 

 preservation is completed. Technically speaking, the silicified cycads are more or 

 less perfect casts of the original tissue systems or "histometabases." The primary 

 chemical reaction resulting in these casts is a separation of silica, at first probably in 

 gelatinous condition, from solutions of alkaline silicates by cellulose. The minor 

 reactions involved must be complex ; but it ought to be possible to silicify complete 

 trunks of existing cycads as well as their foliage and fructifications in laboratories. 



Clearly, then, in dealing with any such highly organized plants as the cycads, 

 it is scarcely to be expected that, even in the case of trunks from the same locality 

 aud of the same species, in themselves originally presenting many individual differ- 

 ences of growth, all these various factors of preservation will have acted in a uniform 

 manner, to say nothing of different localities or geologic periods. Furthermore, the 

 external appearance of specimens will vary greatly not only because of many differ- 

 ences in the nature and extent of their preservation, but because of differences in 

 the process of erosion bringing them within the collector's reach. Later, under the 

 head of trunk structure, will be shown in full detail the wide possibility of variation 

 in external appearance dependent upon the relative position of the zone to which 

 preservation has extended, or at which either secondary chemical action or erosion 

 has ceased. 



Obviously, where it is wished to obtain more than initial knowledge of external 

 features, it is in all cases indispensable to have constant recourse to polished 



