20 INVERTEBRATE PALAEONTOLOGY 



bulk of a calcareous rock is organic in origin, and thus 

 in some measure crystalline, a large proportion is so 

 finely comminuted that it reacts almost as if it were 

 amorphous. Hence the " matrix " of a limestone usually 

 proves more readily soluble in atmospheric water than 

 the shells that it includes, and these are in consequence 

 less likely to be seriously harmed (see PI. i. fig. 2). (If 

 the fossil is made of Aragonite, Opaline silica or Chitin, 

 solution, decay or petrifaction is almost inevitable.) 

 It is usual to find a secondary infilling of calcite, 

 derived from percolating water, occupying all the smaller 

 cavities in the shell-tissue, but this need not obscure 

 the original structure. Again, the matrix consists of 

 material which is theoretically of the same hardness as 

 the enclosed fossils, and which in practice is softer owing 

 to its powdered condition. Hence the pitting that so 

 often defaces shells in sandstones is unlikely to be 

 impressed on those in a limestone. 



Except in mines and borings, fossils can be collected 

 only from rocks exposed at the Earth's surface. Such 

 rocks are always undergoing destruction by weathering 

 agencies, and the fossils they contain must submit to 

 the same fate, however " extraneous " their origin. Thus 

 the most perfect and complete preservation is but 

 temporary in relation to geological time. In many 

 districts sedimentary rocks thousands of feet in thickness 

 have been removed by denudation at various geological 

 periods, and no vestiges of their faunas are preserved, 

 save for occasional water-worn relics embedded in later 

 deposits, or chance occurrences in the pebbles of 

 conglomerates. But the tool of erosion is double-edged, 

 and brings to light fossiliferous rocks that would other- 

 wise remain buried and out of reach. Other and more 

 merciless agents may work havoc on rocks and fossils, 

 without affording a chance for rescue by collection. 



