THE MICROSCOPE IN GEOLOGY. 
359 
titanoferrite, iron pyrites, &c., which last are frequently present 
in so minute a quantity as only to be detected by the microscope. 
Whatever be their geological age, or from whatever part of 
the earth’s surface they be taken, the microscopical inspection 
of such rocks shows immediately that they possess certain 
general and definite structural characters, distinguishing them 
at once from all other rocks. 
The mineral constituents of such rocks are seen to be de- 
veloped as more or less perfect crystals, at all angles to one 
another, thereby indicating that the entire mass must have 
been one time in a state of liquidity or solution (aqueous or ig- 
neous), sufficient to allow of that freedom of motion absolutely 
essential to such an arrangement of the particles.* 
It would be impossible to do justice to this subject without 
more space, and many other illustrations, than are at disposal in 
this communication ; some, however, which will serve to point 
out the general features of the structure of this class of rocks, 
are attempted in PI. XVII. figs. 1 to 8, and PI. XVIII. figs. 
9 to 12. 
The microscopic examination already made of many hundred 
sections of eruptive rocks, differing widely in geological age and 
geographical distribution, shows that in all rocks of this class, 
whether of the most compact, hard, and homogeneous appear- 
ance, or occurring in the softest and finest powder, like the 
ashes and dust frequently thrown out by volcanoes ; a similar 
crystallised arrangement and structure is present and common 
to them all. Lavas, trachytes, dolerites, diorites, porphyrites, 
syenites, granites, &c., all possess the same general structural 
features, serving to distinguish the eruptive rocks as a class from 
all others. 
In the examination and discrimination of the minerals which 
compose these rocks, especially when close-grained, the micro- 
scope is quite indispensable, since without it no such enquiry 
could be attempted. In these examinations, the assistance of 
polarised light is most valuable ; but the space, unfortunately, 
* Experiments show that analogous structure can be produced by at least 
three different methods, all of which, however, agree in the necessity of the 
mass being in a state of complete liquefaction previous to crystallisation ; 
from — 
1. Their solutions in water or other menstrua. 
2. Aqueous fusion or melting of hydrated bodies in their water of 
crystallisation. 
3. Igneous or hydro-igneous fusion. 
Crystalline structure may nevertheless develop itself by a molecular 
movement in solid bodies without change of external form or previous 
liquefaction ; as will be hereafter explained, this is frequently the case in 
nature. The structure so developed is, however, very distinct from the 
crystallisation after liquefaction, characteristic of the eruptive rocks. 
