m 



COLLECTING AND PREPARING FOSSILS SCHUCHERT. 



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 distinguishing 

 characteristics of pseudomorphs. In many xases pseud omorphs were evidently 

 formed by molecular replacement. All those chalcedonic pseudomorphs of shells 

 ■which sometimes occur in limestone, and from which they often may be freed in a 

 complete condition by acids, have doubtless been produced by molecular replace- 

 ment. 



Carbonised fossils. — The term carbonization is applied in this connection only or 

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

 remains are of great economic value and often of great importance in structural geol- 

 ogy, they are of little paleontological importance, because the organic structure of 

 the plants from which they were derived has usually 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. 



Fossil molds. — 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 rocks. Such solvents, while 

 completely removing certain kinds of fossils, sometimes left others unaffected, and 

 sometimes they acted equally upon fossils of essentially the same chemical composi- 

 tion. For example, the shells of the Ostreidte almost always have resisted such sol- 

 vents 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 diff"erent ways ; for example, by compression of the mold after it was formed, or 

 by its having received a drusy lining. 



Imprints of fossils. — Imprints do not differ materially in character from molds, the 

 former term being usually applied to impressions left in the rock by thin substances 

 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 impriuts 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. 



Natural casts of fossils, — Casts are counterparts of fossils, having been produced 

 by the filling of molds with a substance other than that of the original fossil. It 

 may have been by the injection, caused by pressure or otherwise, of substance 

 derived from the matrix or inclosing rock, or by the precipitation of substances 

 brought into the cavity suspended in percolating water. If, in the latter case, the 

 cast is composed of a crystallized mineral, the term pseudomorph is applied to it, as 

 already stated. Natural stony casts of the interior of shells and other fossils are 

 often found within the molds which were formed by the solution and removal of the 

 fossil itself, and they are also often found filling permineralized shells. The student 

 of fossils often finds it desirable to take artificial casts of natural molds, especially 

 in case he can obtain no other representation of the species he desires to study. By 

 such a cast the original form and surface features are often reproduced with the 

 greatest accuracy. * ^ * Although the soft parts of animals could never have 

 become really fossilized, cases have occurred of the preservation in fine sediments 

 of their form and even parts of their structure, in the condition of impriuts or casts. 

 A most remarkable and exceptional case of this kind is that of the jelly-fishes of 

 the Jurassic slates of Solenhofeu [and Middle Cambrian of Alabama], where, in the 

 fine sediments of which the slates were originally composed, not only their shape but 

 the essential parts of their structure are preserved. 



Adipocere bodies. — Fossilization or petrefaction of human bodies is often popularly 

 reported to have occurred, but these are only cases of the change of the adipose and 

 muscular tissues of the body to the wax-like substance adipocere, which process 

 only delays but does not prevent final and complete decomposition. This change 



