LIFE. 615 



The change often consists in the reception of new mineral matter into the pores or 

 cellules of the fossil, as when bones are penetrated by limestone or oxyd of iron. 



The change is frequently a true petrifaction, in which there is a substitution of new 

 mineral material for the original ; as when a shell, coral, or wood is changed to a sili- 

 ceous fossil, through a process in which the organism was subjected to the action of 

 waters containing silica in solution. In other cases, the organism becomes changed to 

 carbonate of lirne, as in much petrified wood; and in others, to oxyd of iron and py- 

 rites; and more rarely to fluor spar, heavy spar, or phosphate of lime. 



The mineral matter first fills the cells of the wood, and then takes the place of 

 each particle as it decomposes and passes away, until finally the original material is all 

 gone. Some fossil logs are carbonized at one end and silicified at the other. 



In many silicified shells, stems of Crinoids, etc., in the Subcarboniferous rocks of 

 Illinois and Indiana, the shell or stem has been split open, and much enlarged, by the 

 infiltrating silica; owing apparently to successive depositions of silica between the 

 shell and the first-formed siliceous layer within the cavity, as the silicifying process 

 went forward. 



The silica in most siliceous petrifactions has come from siliceous organisms associated 

 with the fossil in the original deposit. 



B. Accumulation into Beds. Calcareous remains of organisms, as 

 shells, corals, etc., have very frequently been ground up by the action 

 of waves or by currents of water, and thus reduced to a calcareous 

 earth, — the solidification of which (as explained on p. 619), has made 

 limestones. 



When the fossils are minute, like Rhizopods and Diatoms, the 

 simple concretion of the shells will make a solid rock, as in the case of 

 chalk and flint (p. 478). 



Ehrenberg estimates that about 18,000 cubic feet of siliceous organisms annually 

 form in the harbor of Wismar, in the Baltic; and he has also found that similar accumu- 

 lations are goinc on in the mud of American and other harbors. 



The bed of Rhizopods accumulating in the North Atlantic, mentioned on page 477, 

 contains, according to Huxley, about eighty -five per cent, of these calcareous shells, 

 mostly of the genus Globigerinn, besides some siliceous Diatoms: it has probably a 

 breadth (between Ireland and Newfoundland) of 1,300 miles, and extends at least some 

 hundreds of miles to the south. Ehrenberg found, in a specimen examined by him, 

 eighty-five species of calcareous llhizopods, sixteen of Polycystines, and seventeen of 

 Diatoms, with only a few arenaceous grains not of organic origin. 



The siliceous shells of the microscopic Polycystines have been found not only in the 

 frigid Sea of Kamtchatka (see Amer. Jour. Sci., II. xxii. pi. 1, for figures) and the 

 North Atlantic, but also in the South Pacific, on both coasts of the Atlantic, in the 

 Mediterranean, and, within the tropics, at Barbadoes, in the West Indies, and the Nicobar 

 Islands, in the East Indies. Ehrenberg has named 282 species from a marl-like deposit 

 at Barbadoes, considered as Tertiary, and 100 species from the Nicobar Islands, part of 

 them identical with those of Barbadoes. 



But, when the fossils are comparatively large, as ordinary corals and 

 shells, the intervals between them must be filled with earth of some 

 kind, derived from the wearing action of the waters. It may be the 

 mud or detritus from rivers or from wave-action along sea-shores ; 

 but, when calcareous, it has evidently come from the wear of the 

 shells, corals, or crinoids themselves ; and hence any limestone rock, 

 made up of large shells, corals, or crinoids, which has the interstices 



