
Boog V.] - -FOSSILIZATION. 609 
is silica, which constitutes the frustules of diatoms and the harder 
‘parts of many protozoa, and is found also in the teeth of some 
molluscs. The integuments of insects, the carapaces of crustacea, 
and some other organisms are composed fundamentally of chitin,? 
_a transparent horny substance which can long resist decomposition. 
In the vegetable kingdom the substance known as cellulose 
forms the essential part of the framework of plants. In dry air it 
_ possesses considerable durability, also when thoroughly water-logged 
and excluded from meteoric influences. In the latter condition, 
imbedded amid mud or sand, it may last until gradually petrified. 
It is a familiar fact that in the same stratum different organisms 
occur in remarkably different states of fossilization. This is some- 
times strikingly exemplified among the mollusea. The conditions 
for their preservation may have been the same, yet some kinds 
of shells are found only as empty moulds or casts, while others still 
retain their form, composition, and structure. ‘This discrepancy, no 
doubt, points to original differences of chemical composition. The 
aragonite shells of a stratum may be entirely dissolved, while those 
of calcite may remain (pp. 82, 166). The presence, therefore, only 
of calcite forms does not necessarily imply that others of aragonite ~ 
were not originally present. But the conditions of fossilization have 
likewise greatly varied. In the clays of the Mesozoic formations, for 
example, cephalopods may be exhumed retaining even their pearly 
nacre, while in corresponding deposits among the Paleozoic systems 
they are merely crystalline calcite casts. 
— 2. Fossilization.—tThe numerous forms of fossilization may be 
reduced to three leading types. — 
(1.) The original substance is partly or wholly preserved. Several 
grades may be noticed: (a) where the entire animal substance is 
retained, asin the frozen carcases of mammoths in the Siberian cliffs ; 
(b), where the organism has been mummified by being encased in 
_ resin or gum (insects in amber); (c), where the organism has been 
_ earbonized with or without retention of its structure, as is character- 
istically shown in peat, lignite, and coal; (d) where a variable portion 
_of the original substance, and especially the organic matter, has been 
removed, as happens with shells and bones: this is no doubt one 
of the first steps towards petrifaction. 
(2.) The original substance is entirely removed with retention 
merely of external form.—Mineral matter gathers round the organism 
and hardens there while the organism itself decays. Eventually a 
mere mould of the plant or animal is left in stone. Every stage in 
this process may be studied along the margin of calcareous springs and 
streams (ante, p.461). The lime in solution is precipitated round fibres 
of moss, leaves, twigs, &c., which are thereby incrusted with mineral 
matter. While the crust thickens the organism inside decays, until a 
1 According to C. Schmidt, the composition of this substance is C, 46°64; H, 6°60; 
N, 6°66; O, 40°20. The brown chitin of Scottish Carboniferous scorpions is hardly 
distinguishable from that of recent species. 
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