No.l.] WHITE ON CONDITIONS OF PEESERVATION OF FOSSILS. 139 



lyopomatous brachiopods and Ostracoids, are well preserved, and in such 

 condition as to plainly indicate tlieir chitinons origin. In these shales 

 also the crinoidal and other echinodermal remains exist in their usual 

 condition of crystalhzed lime-carbonate. 



In some of the shales of the coal-measures of Illinois and the adjoining 

 States, however, certain species of Conchifers are as perfectly preserved 

 in a calcareous condition as any of the arthropomatous brachiopods 

 are ; but, as a rule, the shells of Conchifers are not so well preserved m 

 these shales, nor in the paleozoic rocks generally, as those of the bra- 

 chiopods are. It is also noticeable that the shell-substance of the Cei)ha- 

 lopods of the paleozoic rocks is not so completely i)reserved as it is in 

 those of the mesozoic strata. 



WeU preserved mesozoic fossils appear to have undergone, in some 

 respects, a less complete change than those of the paleozoic rocks. For 

 example, the shells of Cephalopods, and some other mollusks, usually 

 retain their original pearly lustre and iridescence, and the fossils gener- 

 ally have a less mineralized aspect than those of the paleozoic rocks. 

 There is, of course, much difference in these and other respects to be 

 observed in the different strata of that age and in different regions. 

 The following remarks relate mainly to the mesozoic strata and their 

 fossils in the western portion of the national domain, and iDrincipaUy to 

 the Cretaceous strata there. 



The mesozoic rocks of the Western Territories consist very largely of 

 sandstones and sandy shales, with occasionally clayey layers, which are 

 usually also more or less sandy. Limestones are rare among those rocks, 

 but the sandstones and shales all contain more or less lime-carbonate, 

 usually in comparatively small proportion, but sometimes enough to be 

 detected without a chemical test. In the mesozoic sandstones it is often 

 the case that all the contained fossils are in the form of casts and moulds, 

 with the not infrequent exception of the Ostreidce alone. It is often the 

 case also that all the fossils of a sandstone are preserved by fossiliza- 

 tion proper, and all in essentially the same calcareous condition, includ- 

 ing the shells of Cephalopods, Gasteropods, and Conchifers among mol- 

 lusks, as well as the few Eadiates and Articulates which occur there ; 

 but it is often noticeable, especially in the Cretaceous sandstones, that 

 the shells of the Ostreidce resist the solvents, to which they have all been 

 exposed, better than those of any other mollusks. Indeed, it sometimes 

 occurs that in sandstones which carry a great variety of molluscan shells 

 in the form of casts and moulds, the Ostreidce alone are completely pre- 

 served in their natural calcareous condition, having evidently ne\-er 

 been materially affected by the solvents which reduced all their asso- 

 ciated molluscan shells to the condition of moulds and casts. In the 

 Cretaceous rocks of the Western Territories, all other fossils than the 

 shells of mollusks are very rare ; and when the latter are well preserved 

 by fossilization proper, so far as the writer's observations have gone, 

 there appears to be very little difference in the condition iii which those 



