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clusively proved that this has not resulted from any chemical or 
crystalline action whatsoever, the particles being in themselves 
perfectly unaltered ; and that the arrangement is solely due to 
the effects of pressure, applied at right angles to the structure 
itself, thereby causing an elongation or flattening out of some, 
along with a sliding movement of other of the particles. The 
amount of compression to which an ordinary roofing-slate has 
been subjected in one direction, has been calculated, approxi- 
mating from the elongation or distortion of the particles, to be 
about equal to one-half of its original volume. 
Besides the cleavage structure, so produced by the compression 
of rocks whilst in a more or less plastic state, Mr. Sorby has 
shown that another system of minute jointing may also be present 
in these rocks, the serrated edges of which, as seen by the micro- 
scope, prove it to have been the result of force applied to the 
rock subsequently to its having been in a perfectly rigiT condition. 
Kocks of this class, when somewhat close-grained and much 
indurated, have not unfrequently, from their external appear- 
ance, been mistaken for intrusive rocks : thus the rock shown in 
section, PI. XVIII. fig. 16, an upper oolitic, highly-inclined 
shale bed, has been mapped by D’Orbigny as an eruptive green- 
stone ; the microscopic structure proves the contrary most con- 
clusively. 
3. Rocks composed of mineral substances extracted from 
aqueous solution by crystallisation , precipitation , or the 
action of organic life. — Under this class are included most 
beds of gypsum, rock salt, and other saline bodies, as well as 
travertine, siliceous sinter, flint, infusorial slates and earths, 
limestones, &c., many of which have been as yet but very 
superficially examined. 
In the microscopic investigation of such rocks as owe their 
origin to the development of organic life, very considerable pro- 
gress has been made, with correspondingly important and in- 
teresting results. 
As early as 1836, Ehrenberg proved that large rock masses 
were built up of the carapaces of minute siliceous infusorise, and, 
more lately, Sorby has done good service by his investigation of 
limestones ; these he has proved not to have originally possessed 
any crystalline structure whatsoever, but to have been deposited 
as mere mechanical aggregates (aptly termed by him, organic 
sands or clays) formed of the debris of calcareous organisms, 
which admit frequently, not only of being recognised, but of 
having their relative proportions determined. The comparison 
of the microscopic structure of the organisms in chalk, with 
those now forming in the depths of the Northern Atlantic 
Ocean, indicates that there is an immense deposit now in course 
of formation, quite analogous to what had previously taken place 
