MATERIALS 29 



represented by obscure impressions. It is only in 

 associated shales, or in mineral concretions, that the 

 Invertebrate fauna of the Coal Measures is adequately 

 preserved. 



(II) FOSSILS AS ROCK-FORMERS (Pis. i. and ii.) 



Organic rocks, as their name implies, consist of 

 matter that owes its solidity and form to the influence 

 of living beings. The mineral substances most abun- 

 dantly secreted by Invertebrates are Calcium carbonate 

 and phosphate, and colloid Silica. Since these minerals 

 are readily soluble in crustal water, considerable mole- 

 cular rearrangement is bound to supervene in rocks built 

 of them. Hence the organic origin of a rock does not 

 automatically imply persistence of recognizable organic 

 remains. Solution and re-deposition of Calcite, and 

 conversion of opal into Chalcedony, will entirely 

 obliterate small shells, and destroy the finer features of 

 larger ones. The most extensive and massive deposits 

 of calcareous and siliceous rock are the oozes, whose 

 chief constituents are small Protozoa. In such deposits 

 as Chalk or Barbados Earth, Foraminifera and Radio- 

 laria, however minute, are in preservation equal to that 

 of modern ooze-formers. But in limestones of the 

 texture of the Carboniferous Limestone, or cherts like 

 those of the Scottish Ordovician and Devonshire Culm, 

 Protozoa are usually scarce and indistinct. It is note- 

 worthy that the few Foraminifera known from Palaeozoic 

 rocks are, on the whole, larger than those characteristic 

 of more recent deposits ; while the surviving Radiolaria 

 in Culm-Measure cherts are often distinguishable with 

 the naked eye. This suggests that the smaller organic 

 remains of ancient deposits have disappeared during 

 molecular readjustment, and that Protozoan oozes may 

 actually have been as abundant in Palaeozoic times as 



