RELATION OF BIOLOGY TO GEOLOGICAL INVESTIGATION. 265 



Pseudomorphism <>!' fossils is so nearly like thai of mineral crystals 

 that this term is equally applicable to both. It consists in the replace- 

 ment of the original substance of the fossil by a crystallizable or crys- 

 tallized mineral, such, for example, as calcite, pyrite, quartz in the form 

 of chalcedony, etc., the original form of the fossil being perfectly retained. 

 It is evident that at least a part of the crystallized pseudomorphs 

 were formed by the precipitation of the component mineral from its 

 solution within such cavities as are described as molds in another 

 paragraph. In such cases they differ from casts as described on the 

 next page only in being crystallized, but crystallization is one of the dis- 

 tinguishing characteristics of pseudomorphs. In many cases pseu- 

 domorphs were evidently formed by molecular replacement. All those 

 chalcedonic pseudomorphs of shells which sometimes occur in lime- 

 stone, and from which they often may be freed in a complete condition 

 by acids, have doubtless been produced by molecular replacement. 



The term carbonization is applied in this connection only or mainly 

 to such masses of vegetal remains as coal, lignite, and peat. While 

 such remains are of great economic value and often of great importance 

 in structural geology, they are of little paleontological importance, be- 

 cause the organic structure of the plants from which they were de- 

 rived has been so completely obliterated as to render them useless for 

 such a purpose. Occasionally, however, fruits and other separate 

 parts of plants are found to have acquired a carbonized condition in 

 which their botanical character may be approximately determined. 



Molds are cavities in sedimentary rocks which were originally occu- 

 pied by fossils, the latter having been subsequently removed by the 

 percolation of water containing a solvent of the fossils but not of the 

 rock. Such solvents, while completely removing certain kinds of fos- 

 sils sometimes left others unaffected, and sometimes they acted un- 

 equally upon fossils of essentially the same chemical composition. For 

 example, the shells of the Ostreidae almost always have resisted such 

 solvents more than have most other shells. The original surface features 

 and markings of fossils are often minutely preserved in molds, but 

 they are frequently obscured in different ways; for example, by com- 

 pression of the mold after it was formed, or by its having received a 

 drusy lining. 



Imprints do not differ materially in character from molds, the former 

 term being usually applied to impressions left in the rock by thin sub- 

 stances like leaves of plants, wings of insects, etc., after their removal 

 by decomposition. Sometimes, however, the molds of shells and other 

 fossils have been reduced to the character of imprints by the extreme 

 pressure to which the strata containing them have been subjected. The 

 details of imprints have often been obscured by pressure, as in the case 

 of molds, but they are often preserved with the greatest degree of 

 minuteness. 



