Alumine frequently also enters into the composition of 

 fossils, as well as of their containing matrices ; clay fre- 

 quently fills their cavities, and forms the beds in which they 

 are found. Bituminous shale, slate, and argillaceous iron 

 stone, often contain fossils, Fullers'-earth has sometimes, 

 though rarely, been found to contain some particular fossils. 

 The nuclei of fossil shells have been formed of hornblende, 

 and both vegetable and animal fossils have been found in 

 substances designated as basalt, wacke, and trap. 



The metals which most commonly contribute to the for- 

 mation of fossils are iron and copper, rarely lead or zinc, and 

 still more rarely silver ; they generally exist in the form of 

 carbonates or sulphurets. The carbonates as well as the 

 sulphurets of iron are chiefly found in vegetable remains. 

 Ligneous fossils of this species present a very curious fact : 

 although the texture of the wood appears to be nearly un- 

 altered, its substance has been so intersected by the chrys- 

 tallizations, that by spontaneous decomposition it is resolved 

 into an impalpable efflorescence. The specimens of wood 

 which have been mineralized by copper sometimes possess a 

 considerable degree of beauty, from the brilliant colours of 

 the malachite which enters into their composition. The 

 sulphuret of lead, galena, has been found in fossil wood ; and 

 blende, the sulphuret of zinc, has been found, with quartz 

 crystals, investing fossil shells. Silver is said to exist in a 

 fossil somewhat resembling the ears of corn, found in the 

 mines of Frankenberg, in Hesse. 



Casts or nuclei of organic remains are formed by different 

 mineral substances filling their cavities, and thus taking the 

 impressions of their internal forms and markings. Impres- 

 sions of the external surface are formed by investment by 

 the surrounding matrix and by its subsequent induration. 

 After this is accomplished, and the original substance re- 

 moved, a cavity or mould is left in the matrix corresponding 

 in its figure and markings with the removed substance. 



