io82 PRINCIPLES OF STRATIGRAPHY 



sphalerite, galenite, and marcasite are also known. Galenite has 

 been reported as replacing the fronds of ferns in some Coal Pleas- 

 ures of Saxony, Malachite, azurite, and chalcocite are found in 

 carboniferous marls, probably of Jurassic age, in the district of An- 

 gola, West Africa, in the Urals, and in other regions. Even mod- 

 ern cedar wood has been found coated wdth and, in some cases, 

 largely replaced by malachite, as reported by Dr. A. F. Rogers 

 from Brigham, Utah (21). From a tufif bed enclosed between 

 basaltic tiows below the Limburg (Germany) wood of Primus 

 nadiis (?) replaced by a kaolin-like substance has been obtained in 

 abundance. This still retains the structure and occasionally car- 

 bonaceous remnants of the wood occur. 



\\'here the actual plant remains have been removed by decay an 

 impression or mold often remains, in which a cast of the plant may 

 be formed by infiltrating foreign material. Such casts are common 

 in the Carbonic sandstones of the Joggins region of Nova Scotia, 

 in western Scotland and elsewhere. Trunks of Calamites, Sigillaria 

 and Lepidodendra, together with their rootstalks. Stigmaria, are 

 abundant in these strata as sandstone casts, resulting from the filling 

 of the cavities left by the decaying wood. 



Decaying wood or other delicate parts of plants may leave a 

 record behind in the rocks in the form of a film of colored mineral 

 matter, precipitated by the decaying organic matter, or by the re- 

 moval of the coloring matter of that portion of the rock covered 

 by the decaying plant. This process of self-inscription upon the 

 rock by the plant has been termed autophytograpJiv (White- 28), 

 the first mode producing a positive picture, and the second a nega- 

 tive one. 



Petrifaction of mineral structures. 



(a) Protozoa. The shell of the Foraminifera is typically com- 

 posed of carbonate of lime, either in the form of calcite (vitreous 

 species) or of aragonite (porcellaneous species). The skeletal 

 structures of Radiolaria are mainly of silica, though horny types (of 

 acanthin) also occur. Both types of Protozoa are well adapted for 

 preservation and extensive deposits of them are known, such as the 

 Radiolarian beds of Barbados (jMiocenic) and the chalk of west- 

 ern Europe (Cretacic). 



(b) Sponges and hydrocoans. These organisms are generally 

 capable of preservation on account of the chitinous material which 

 composes the netAvork of many sponges and forms the perisarc of 

 the Hydrozoa. They are most commonly preserved as carbonaceous 



