m.%] WHITE ON CONDITIONS OF PEESEEVATION OF FOSSILS. 137 



of cMtinous origin when they are compared with fossils of calcareous 

 origin. Chitiuoiis fossils are also among the most permanent of fossil 

 forms, and they are often found in a perfect condition, when all their 

 associated fossils, which were originally calcareous, exist only in the 

 form of casts or moulds. This is especially noticeable in the case of 

 lyopomatous brachioi)ods in sandstones and some magnesian limestones. 

 Although the mineral composition of calcareous fossil shells and corals 

 is usually that of almost pure calcite, it is seldom, if ever, the case that, 

 upon fracture, they show the j)lanes of cleavage of that mineral; hut 

 they generally, as before stated, preserve their original microscopic struc- 

 ture. On the contrary, in almost all cases, all x)arts of every fossil ech- 

 inoderm shows upon fracture the true crystallized, form of calcite. The 

 mineral is evidently nearly pure, but it is never transparent and seldom 

 translucent, except in thin ilakes along the cleavage planes. It is also 

 an interesting fact in this connection that while the substance of every 

 one of the numerous and often minute pieces which compose the body 

 and appendages of fossil echinoderms of all kinds, may be so perfectly 

 crystallized as to allow free cleavage upon all sides of the fundamental 

 rhombohedron of calcite, none of the planes of cleavage ever pass from 

 one joint or piece to another, even though the parts may be not only in 

 their natural position of contact, but solidly minerahzed together. The 

 IDcrfect mineralization of echinodermal bodies, and almost invariably in 

 the form of calcite, appears the more remarkable when it is remembered 

 that the substance of those bodies in the living state is much less solid 

 than that of any moUuscan shell. Possibly this porous character, admit- 

 ting the addition of a greater proportion of calcite in solution by percola- 

 tion or saturation than could enter the more solid shells, affords the true 

 ground for explaining the phenomenon just noted, but it still remains 

 nnexiilained why the granular texture of the original substance of the 

 echinodermal body is always obliterated, and why the sutures between 

 the parts always form perfect barriers to the continuation of the planes 

 of cleavage. These ijlanes are not only interrupted by the sutures, but 

 their direction is always different in each piece, showing that the jjrocess 

 of crystallization was independent in each piece, however small it may 

 have been. 



Although calcareous fossils are fully preserved in the purer and ordi- 

 narily comjiact limestones, they have in such cases become so compacted 

 with the stone itself that it is always diflicult and usually imj^ossible to 

 arrive at a full knowledge of their structure and characteristics. It is, 

 therefore, in strata which are only partly calcareous that invertebrate 

 fossils are, as a rule, the most completely preserved. In these cases, the 

 fossils seem to have served as nucJei to which was attracted a sutticient 

 amount of lime-carbonate in solution to sohdify them by its precipita- 

 tion within tlieir substance and interstitial cavities, while the imbed- 

 ding rock was not greatly hardened either by that or any other process. 

 Therefore, the most perfect calcareous fossils are found in the clayey 



