SPECIAL MODES OF PRESERVATION 1089 



exception of a small space near the periphery was filled solidly 

 with a beautiful crystalline mass of the mineral colemanite. 'Tn 

 several places next the shell a semi-fluid layer of bitumen occurs, 

 which probably represents the original organic matter within the 

 shell." 



Excessive silicification. 



In some cases, notably in the Carbonic strata of the Mississippi 

 Valley region, it has happened that silica has been deposited to 

 excess in crinoids, shells or corals, with the result that the original 

 form has been wholly destroyed, the fossil at the same time swell- 

 ing out enormously. This excessive deposition goes on more par- 

 ticularly along lines of fracture, or, as in the crinoid calices, 

 between the component plates. These become more and more 

 separated as the deposition of silica goes on, and they also become 

 sunken below the level of the network of silica. Eventually they 

 are probably buried in the accumulation of silica which closes over 

 them. Thus from a small fossil in which all the plates are discerni- 

 ble, a large mass of structureless silica is formed, which seldom 

 gives a clue to its origin. (Bassler-i.) 



Molds and Casts. Whenever organisms are buried in material of 

 sufficient plasticity to adapt itself to the contours of the buried 

 bodies, molds of the exteriors of these bodies will be made. Such 

 molds may be temporary or they may be persistent ones. Lutace- 

 ous material generally furnishes the most perfect molds. Even soft 

 tissues, when encased in a matrix which solidifies rapidly enough, 

 may leave a mold behind after decay. Thus the bodies of human 

 beings buried in the volcanic mud which overwhelmed Herculaneum 

 and Pompeii left behind a perfect mold of their exterior. Again, 

 the calcareous tufa, forming constantly in many portions of the 

 earth, encloses leaves, mosses, or even fish and other animals and 

 covers them with a crust of lime. On the subsequent decay of 

 the enclosed body a perfect mold of its exterior is commonly pre- 

 served from which an artificial cast could be made. Viscous lava 

 also may flow around and enclose a foreign body, which, if it is 

 able to withstand the heat of the molten mass, may leave a distinct 

 mold or impression. Impressions of medusse are known from the 

 Cambric of the southeastern United States (Walcott-25). When 

 two valves of a bivalve mollusc become buried in juxtaposition, the 

 space between them is filled with mud and thus an internal mold 

 is produced. The same occurs in gastropods, in cephalopods and 

 in other shelled animals, and may also be found in trilobites and 



