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Records of the Geological Surrey of India. 
[VOL. x. 
blocks of mica-schist caught up by and buried in the granite. They are of all shapes, and 
varied in diameter from 2 inches to 2 feet. These blocks are identical in appearance and 
composition -with the mica-schists through which the granite passes, and^.cannot, I apprehend, 
be due to segregative action. 
IX.—Microscopical chaeactebs of the Gneiss. 
In connection with the foregoing field observations, I have prepared many (over 
220) thin slices of rocks for examination under the microscope. Latterly I have turned 
my attention to the central gneiss. Speaking generally, the internal structure of the rock— 
particularly when it passes into a granitoid state—is that which has usually been described 
as characteristic of an igneous rock.* The crystals of felspar and quartz contain within 
them micro-crystals pointing in all directions. Liquid cavities are often numerous, and they 
frequently contain bubbles that move about restlessly, similar to those described by Sorby and 
others-t 
Sorby has shown in his paper on the microscopic structure of crystals (Yol. XIV, 
Q. J. G. S., p. 453) that those bubbles have been formed by the “ contraction of the fluid on 
cooling.” Air and gas bubbles are readily distinguished under the microscope from vacuum 
bubbles. 
I note the presence of these fluid cavities and bubbles, because I wish to draw from this 
fact the inference that the central gneiss has been subjected to the influence of heat. This 
influence may, I think, be drawn from the cavities I have observed in these rocks. These 
cavities appear to have been filled with a mixture of steam or highly heated water, and air 
or gas, and the two substances have separated on cooling. 
That the heat was very great, and reduced the rock to a plastic condition, may be inferred 
from the presence of what Sorby calls glass cavities. In these it is seen that the glass or 
mineral matter formed contraction-bubbles on cooling, similar to those in liquid cavities; 
only, in the case of glass cavities, the bubbles are never movable, there are often more than 
one of them in the same cavity, and they are not always spheres. Frequently, then, their 
shape conforms to that of the crystal or glass cavity in which they are contained. 
Sorby remarks of glass and stone cavities: “ Independent of the fact that in all essential 
characters they are identical with the crystals in artificial furnace slags, their very nature 
proves the igneous origin of the minerals containing them. This is especially the case with 
glass cavities, for nothing but igneous fusion could so liquefy the enclosed glass that perfectly 
spherical bubbles could be produced.” 
The presence of cracks in micro-crystals, where the cracks have not extended into the 
matrix, as occurs in several qf my specimens, is also, I think, good evidence of the rock having 
been subjected to great heat. Subsequent to the cracking the pieces have been severed and 
floated to some little distance from each other. This proves that the matrix was in a limpid 
condition and flowed in between the fractured ends so as to leave no trace of the disturbance. 
All these cracks were, I apprehend, caused by unequal tension either on the cooling or 
re-heating of the mineral. In one case the fractured pieces (one of which contains a 
bubble) appear to have lost the sharpness of their outline by re-heating, whilst two pieces 
have been soldered together. 
* I do not mean to imply by this either that the oek s an intrusive one that this structure cannot be 
produced otherwise than by dry heat. The fact noted mplies that, from whatever cause, the molecules had perfect 
freedom of motion. 
t There is, unfortunately, neither time nor means to reproduce the excellent drawings of these objects 
sent by Colonel McMahon.—H. B. M. 
