THE MICROSCOPE IN GEOLOGY. 
365 
in the seas of the cretaceous period ; and the same able observer 
has shown that the reason why certain calcareous organisms are 
found so well preserved, whilst others had disappeared or become 
entirely disintegrated, was from the carbonate of lime in the 
first being in the form of the stable calcite, whilst in the latter 
it was present as instable Aragonite. 
When a calcareous rock has undergone cleavage, the micro- 
scope shows a distortion of its particles and organisms, just as in 
a cleaved slate, though in much less degree; the measurement 
of such distortion serves as a basis for estimating the amount 
of compression undergone. 
With the exception of having briefly referred to the alterations 
in igneous rocks, subsequent to their solidification, and the 
cleavage of sedimentary beds, all the classes of rocks treated of 
have been considered in their normal or unaltered condition ; it 
remains now to direct attention to the use of the microscope in 
the study of subsequent alteration or metamorphism of rocks. 
Many sedimentary beds become more or less indurated, at 
points where they are cut through by eruptive dykes ; thus the 
coal-shales and clays of Staffordshire are found altered into a 
hard rock with conchoidal fracture, or even into porcellanite, 
when in immediate contact with basaltic dykes. An examination 
shows no change in mineral or chemical composition beyond the 
expulsion of the water always contained in such beds, and 
sections of such rocks are often seen to be quite identical in 
structure with those of common stoneware made from the same 
clays, the only difference being that the latter is usually more 
porous from not having been submitted to the pressure which 
rocks baked in situ would experience. 
The alteration of rocks produced by infiltration may or may 
not be accompanied by chemical changes ; thus a section of 
calcareous grit often shows that the calcite filling up the inter- 
stices between the grains of sand has been merely deposited from 
a solution of carbonate of lime which has percolated through it, 
and in otherwise unaltered limestones it is common to find 
microscopic veins of calcspar, due to minute cracks or fissures, 
filled up in a similar manner. Frequently, however, such infil- 
tration is accompanied by an entire change in the chemical 
composition of the rock itself ; thus the beds of Cleveland iron- 
stone have been proved by Sorby’s microscopical researches to 
have been originally shell limestones converted into carbonate 
of iron by the action of ferruginous solutions, the fragments of 
the original shells being still distinguishable in all stages of 
conversion ; in the same manner he has proved the magnesian 
limestones of the Carboniferous and Devonian ages, as well as the 
Permian dolomites, to have been originally common limestones, 
or aggregations of organic debris, the particles of which, by the 
