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POPULAR SCIENCE REVIEW. 
nature of the material from which each particular rock was 
derived. Some beds are mainly composed of definite fragments, 
so as to be analogous to sands, and then the true nature of the 
various organisms from which the fragments are derived can be 
ascertained, provided they were originally calcite ; whereas, if 
they were originally aragonite and their structure is lost, very 
often all that can be said is that they were portions of aragonite 
shells or corals. Many associated beds are or were composed 
of fine granules, and analogous to clays. In many cases these 
have in all probability been derived to a great extent from 
aragonite organisms decayed down into small granules of 
calcite, and it is quite impossible to further identify the 
material. 
The structure and origin of oolitic grains were dwelt upon at 
some length. Usually they are evidence of true chemical 
deposition, since they are of simple crystalline growth, quite 
independent of living organisms. They occur of three distinct 
types, viz. : those composed of aragonite, like the Carlsbad 
Sprudelstein , having a concentric structure without any radii, 
giving rise with polarized light to a black cross optically 
positive ; those which are composed of calcite, having a radiate 
structure, and giving rise to a negative black cross, which are 
the predominant and characteristic type of our oolitic rocks ; 
and those which have recrystallized since their original forma- 
tion, which in all probability were originally aragonite, subse- 
quently changed into calcite. 
After describing the chief points of interest connected with 
the leading limestone rocks of our country, Mr. Sorby collected 
together the results into two tables, the more condensed 
of which may here be given : — 
Chief constituent fragments, &c., in descending order. 
. Shell prisms, Foraminifera, Coccoliths. 
. Freshwater aragonite Mollusca, Entomostraca. 
{ Chemical deposits, aragonite Mollusca and Corals, Bra- 
chiopoda, Echinoderms, shell prisms. 
. Original structure lost by dolomitization. 
{ Encrinites, Brachiopoda, Foraminifera, Corals, and 
Polyzoa. 
. Encrinites, Corals, and allied organisms. 
. Encrinites, Corals and Polyzoa, Brachiopoda, Trilobites. 
f Original structure lost, quartz and silicates formed 
* \ in situ. 
He concluded as follows : — 
“ On examining these tables, especially the more detailed 
one, it will be seen how remarkably and characteristically our 
limestones differ from one another. There would usually be 
little difficulty in deciding the general age of any characteristic, 
somewhat coarse-grained, specimen. Though this difference 
must to a great extent have depended on the nature of the 
Name of rocks. 
Cretaceous . 
Wealden , . 
Jurassic . . 
Permian . . 
Carboniferous 
Devonian . * 
Silurian . . 
Metamorphic. 
