MATERIALS 37 



crystalline properties. Thus the nucleus of a chalybite 

 or calcareous nodule is often a calcite shell, while that 

 of a flir.t-nodule is usually a siliceous sponge. Although 

 the fossil causing the nodule may have been a mere 

 fragment, it is usual to find organic remains in concre- 

 tions better preserved than those in the surrounding 

 rock. The superior hardness and impermeability of the 

 subcrystalline material give protection to its contents. 

 Thus in the Lias of Whitby, Ammonites in nodules are 

 often perfect in form and detail ; while those in the shale 

 are flattened almost beyond recognition. Similarly, 

 Sponges in the Chalk (PL ii. fig. 2) are represented by 

 obscure impressions, save for those encased in flint. In 

 the latter case, spicules, and ooze-forming organisms 

 mingled with them, can be picked from the " flint-meal " 

 perfect and unchanged. 



Marcasite and Pyrites are often deposited in crystal- 

 line masses around fossils buried in clay. Commonly 

 the enclosed organisms have been petrified by the 

 mineral, precipitation continuing after replacement was 

 complete. More rarely the original shell is retained 

 with an incrustation of Iron Pyrites showing a rough 

 indication of the form of the fossil (PI. v. fig. 2). Such 

 nodules are not satisfactory from the palaeontological 

 standpoint, since the hardness of the crust, and its 

 liability to decay with liberation of sulphuric acid, make 

 extraction and preservation of the contents difficult or 

 impracticable. 



(V) DERIVED FOSSILS 



Sedimentary rocks are built of the ruins of others. 

 The materials employed may have formed part of 

 igneous, metamorphic or older sedimentary rocks. The 

 ruins of a sedimentary rock are likely to include some 

 of its enclosed fossils. Hence it follows that fossils 



