266 ■ REPORT OF NATIONAL MUSEUM, 1892. 



Casts are counterparts of fossils, haviug been produced by the tilling 

 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 precipi- 

 tation 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 pseudoinorph 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 re- 

 moval of the fossil itself, and they are also often found filling permineral- 

 ized shells. The student of fossils often finds it desirable to take ar- 

 tificial 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 great- 

 est accuracy. 



In the foregoing paragraphs are described the principal conditions 

 in which fossils occur or by which they are represented, but one occa- 

 sionally finds specimens which indicate certain conditions that are not 

 fully recognized in the foregoing descriptions. 



These cases, however, are less important than are those which have 

 been considered, and they need not be further mentioned, but it is 

 desirable to refer to certain conditions under which the soft parts of 

 animals have sometimes been represented by impressions in sedimentary 

 rocks, or under which they have been preserved from ordinary decay 

 for an unusual length of time. 



The fact was referred to on page 252, that 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 imprints or casts. A most remark- 

 able and exceptional case of this kind is that of the jelly-fishes of the 

 Jurassic slates of vSolenhofen, 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. Impressions, presumably of 

 similar animals, have been found in older rocks, but these are less per- 

 fect than the Solenhofen specimens. 



Fossilization or petrefaction of human bodies is often popularly re- 

 ported 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 chauge frequently occurs in other ani- 

 mal bodies that have become buried in wet or constantly damp earth, 

 and packages of pork recovered from old river wrecks have often been 

 found to have undergone the same change. 



Every specimen of fossilized man is really only a skeleton, but the 

 wonderful cases of preservation of the human form in the partially 

 hardened volcanic ash of Pompeii are worthy of mention in this con 



