io86 PRINCIPLES OF STRATIGRAPHY 



tianite (Sandberger). In the Zechstein of Altenburg shells of 

 Schizodus have been found replaced by malachite, which fills the 

 space between external and internal mold. Gypsum has also oc- 

 casionally served as replacing substance of pelecypods and gastro- 

 pods. Barite is not an uncommon replacing agent of molluscan as 

 well as brachiopod shells and other hard structures. In the Lias 

 of Whitby, England, the ammonites are commonly replaced by 

 barite, colored brownish by bituminous matter. Celestite or angle- 

 site more rarely takes the place of barite. \'ivianite not uncom- 

 monly replaces the guards of IJelemnites in the New Jersey Creta- 

 cic, as well as shells of other molluscs. Even wulfenite has been re- 

 corded as replacing the shell of an Isocardia. Iron pyrite or marca- 

 site is a common replacing agent of mollusc shells, especially those of 

 ammonites. This substance often becomes altered to limonite, and 

 not infrecjuently disintegrates altogether, where not protected from 

 the air; and thus beautiful fossils are destroyed. Blum records the 

 case of an Avicula in which the outer surface was pyrites and 

 the inner calcite. Sphalerite, and, rarely, smithsonite, galenite. and 

 other metallic salts, replace the shells of pelecypods and gastropods 

 and, more rarely, of cephalopods. In the Cote-d'Or, a Liassic 

 pelecypod has been found completely replaced by specular hema- 

 tite, while ordinary red hematite is not infrequently found replacing 

 mollusc shells. Chlorite has been found replacing shells in several 

 cases. In the Tertiary beds of Aragon, Spain, Planorbis has been 

 found replaced by native sulphur (Blum-7:777, i'/6) ; and Tertiary 

 Helix from near Madrid has been reported replaced by meerschaum 

 or sepiolite. 



(f) Crustaceans, Alcrostomes, Insects, etc. As has been noted 

 above, the exoskeleton of Crustacea is composed of chitin impreg- 

 nated with calcium carbonate and phosphate. Sometimes the chitin 

 carbonizes and a black mass of carbon mixed with lime remains, 

 which is susceptible of a high polish. Again, the organic matter may 

 be entirely removed and replaced by calcium carbonate or other min- 

 erals. Thus trilobite tests are sometimes changed entirely to 

 crystalline calcite. Pyrite not infrequently replaces the tests of tri- 

 lobites, as in the famous specimens of TriartJirus becki from near 

 Rome, New York, discovered by Valient, in which Matthew and 

 Beecher have found the antennae and legs beautifully preserved, the 

 whole test having become pyritized. The exoskeletons of mero- 

 stomes correspond closely to those of Crustaceans. The insect body 

 is rarely preserved, except the wings and the elytra of beetles, owing 

 to the absence of mineral matter. 



(g) Echinoderms. This class of animals is characterized by the 



