138 BULLETIN UNITED STATES GEOLOGICAL SURVEY. [Voir. 



and shaly partings of limestone strata and in sandy and clayey shales 

 which are also calcareous. 



It seems diflGLcult to explain why the segregation of lime-carbonate 

 should have ceased at the full solidificatiou of the fossil itself, and why 

 it did not continue until at least the immediately surrounding portions 

 of the imbedding matrix were also hardeneu, with the fossil as a inicleus. 

 This latter condition does sometimes occur; but, as a rule, to which there 

 are only rare exceptions, the matrix is no more fully charged with 

 lime-carbonate, and no harder from any cause, in immediate contact 

 with the fossils, than it is in other x)arts of the same stratum, even when 

 the rock contains enough of lime-carbonate to have thus x)reserved many 

 more fossils than it has ever contained. So completely are the fossils 

 thus preserved, and so distinctly seijarate are they from the matrix, that 

 it is often the case that not only their delicate surface-markings, but 

 their minute and fragile appendages also, are so perfectly preserved 

 that they may be as easily studied as the Corresponding parts of living 

 forms. 



That there should be a difference in the manner and completeness, or 

 otherwise, of the preservation of fossils which were originally different 

 in their mineral composition is too evident to excite remark ; but one 

 would not a priori exx)ect to find any material difference in the complete- 

 ness of the preservation of shells and other fossils which were originally 

 nearly or quite identical in the original composition of their mineral 

 constituent, and that were fossilized under identical circumstances of 

 environment. Such differences, however, do occur, and they are iriore- 

 over as great between certain forms which are zoologically nearly re- 

 lated, as they are between certain others which are distantly related. 

 Thus, for example, the difference in this respect is greater in some in- 

 stances between the shells of different families of the same class of mol- 

 lusks than it is between those of some mollusks and the shells of certain 

 worms. These remarks will be closed by citing a few exam^des of this 

 unequal condition of preservation of invertebrate fossils under identi- 

 cal circumstances of environment. 



Argillaceous shales which are also more or less calcareous i^revail in 

 all the x>aleozoic formations of the eastern portion of I:^ orth America ; 

 and for our present purpose we may select the Hamilton Shales of j^eAV 

 York as furnishing an example of uniformity of circumstances of envi- 

 ronment during the fossilization of an invertebrate fauua. In these 

 shales, the abundant and diverse forms of arthropomatous brachiopods 

 are beautifully and perfectly preserved in almost all cases, and the cal- 

 careous character of the fossils is uniform and perfect. The same may 

 be said of the Corals and Polyzoa ; ^iiile in the same layers which con- 

 tain these fossils, the associated shells of all the Conchifers, without ex- 

 ception so far as I am aware, exist only in the form of moidds and casts, 

 the substance of the shells having entirely disappeared. Associated 

 chitinous fossils, as represented by the crusts of trilobites and shells of 



