MELLO: MICROSCOPICAL STRUCTURE OF ROCKS. 155 
matter. Many years ago, Dr. Sorby, F.R.S., published a very import- 
ant paper "On the Microscopic Character of some Crystals", a paper 
which, more than anything else, gave the impetus to microscopic 
research in geology. In this Dr. Sorby showed that these cavities 
afford a clue to the history of the crystal in which they occur. 
Before giving some of his principal results, it will be necessar}^ to 
point out how these various kinds of cavities can be distinguished 
from each other. When a moderately high power, say i inch, is 
brought to bear upon them, some of them will be found to contain 
a small moveable bubble, which will change its position as the slide 
is turned, or in some cases will be in constant motion; this cavity 
is a "fluid cavity", and when such a bubble is absent we may know 
a full fluid cavity from a gas cavity by the broad black outline of 
the latter, by transmitted light, and also by the fact that the latter 
will shine brilliantly when the light is reflected from its surface, a 
fluid cavity under reflected light being almost invisible, Glass 
cavities sometimes contain bubbles, but these are immovable, stone 
cavities are those which are filled with minute crystals. 
It is of great importance in studying the history of a rock to pay 
close attention to the nature of the cavities found in its component 
crystals. The principal points, as deduced from Dr. Sorby's observa- 
tions, may thus be tabulated : — crystals containing 
Fluid cavities — were deposited from aqueous solution. 
Olass or stone cavitips — were deposited from igneous fusion. 
Oas cavities — were formed by sublimation, or by the solidifica- 
tion of a fixed homogeneous substance, unless they are merely empty 
fluid cavities. 
Fluid and gas cavities in a single specimen — were formed under the 
alternate pressure of the liquid and a gas. 
Fluid and glass oi^ stone cavities in a single specimen — were formed 
under great pressure by combined actions of igneous fusion and water. 
Fluid and gas, and glass or stone cavities in a single specimen — show 
the action of gaseous sublimation, together with igneous fusion and 
a(iueous solution under gi-eat pressure. 
It may be added that the more numerous the cavities, the more 
rapidly was the crystal formed. 
