
106 7 GEOGNOSY. | BBGORER? <4 
Detritus derived from the comminution or decay of organic 
remains presents very different and characteristic structures.1 Some- 
times it is of a siliceous nature, as where it has been derived from 
diatoms and radiolarians. But most of the organically derived 
detrital rocks are calcareous, formed from the remains of foraminifera, 
corals, echinoderms, polyzoa, cirripedes, annelides, molluses, crustacea 
and other invertebrates, with occasional traces of fishes or even of 
higher vertebrates. Distinct differences of microscopic structure 
can be detected in the hard parts of some of the living representatives: 
of these forms, and similar differences have been detected in beds of 
limestone of all ages. Mr. Sorby, in the paper cited below, has shown. 
how characteristic and persistent are some of these distinctions, and 
how they may be made to indicate the origin of the rock in which 
they occur. There is an important difference between the two forms, 
in which carbonate of lime is made use of by invertebrate animals; 

Fic, 14.—Cuastio Srrucrure oF OrGanic OriciIn—Srrvcture or CHALK (Sorsy). 
MaenirieD 100 Diamrrers. (See p. 168.) 
aragonite being much less durable than calcite. Hence while shells 
or other organisms formed largely or wholly of:.axagonite crumble 
down into a mere amorphous mud, pass into crystafline calcite, or 
disappear, the fragments of those consisting of ‘caleite may remain 
quite recognizable, 
It is evident therefore that the absence of all trace of organic 
structure in a limestone need not invalidate an inference from other 
evidence that the rock has been formed from the remains of organisms. 
The calcareous organic débris of a sea-bottom may be disintegrated 
and reduced to amorphous detritus by the mechanical action of waves 
and currents, by the solvent chemical action of the water, by the 
decay of the binding material, as of the organic matter of shells, 
or by being swallowed and digested by other animals.? 
' The student who would further investigate this subject will find a suggestive and 
luminous essay upon it by Mr. Sorby in a recent presidential address to the Geological 
Bociety. Quart. Journ. Geol. Soc, 1879, 
2 Sorby, loc. cit, 
