Royal Society of Edinburgh. 145 
mineral constituents. On extending his observations to obsidian, 
marekanite (a volcanic glass from Lake Marekan in Kamtschatka), 
and also to the well-known glassy obsidian of Bohemia, he found 
they all exhibited a structure analagous to the pitchstones of Arran. 
He further found that sections of glass slags, where the heat had 
been long continued, combined with slow cooling, all presented the 
same appearances as the sections of pitchstone. | 
This structure, peculiar to igneously formed substances, he found 
usually to radiate in a stellate form; and though many slags 
showed large stars visible to the naked eye, the stellate structure is 
more easily observed by the aid of the microscope. The character 
is so marked that no one whose eye is tutored to microscopic observa- 
tion can fail to recognise at once a mineral substance of igneous origin. 
In granite, on the other hand, the structure, as seen by the mi- 
croscope, is as persistent as in pitchstone, glass, and obsidian, but 
totally different. 
In the many experiments which the author had tried with gra- 
nites from various localities, he had never succeeded in obtaining 
one instance of stellate structure, while the constant occurrence of 
cavities containing fluids convinced him that, if pitchstone and 
glass are types of igneous-formed substances, granite must be of 
aqueous origin. In the fluid cavities so abundant in topaz, Cairn- 
gorum, beryl, tourmaline, and felspar, all constituents of granite, he 
found the same appearance prevailed. These cavities are seldom 
entirely filled with fluid, an air-bubble usually occupying more or 
less of the cavity. After many hundred experiments on such 
cavities, the author found that when exposed to a tetperature of 
94° Fahr., the bubble disappeared, the fluid entirely filling the 
cavity, and at the temperature of 84° the bubble reappeared with a 
singular ebullition, showing that the air had formed an atmosphere 
round the fluid. He was thus led to infer that those cavities could 
not have been filled at a temperature above 84°, and certainly not 
above 94° of Fahrenheit. 
As another proof that these cavities could not have been filled 
when the temperature of the surrounding rock was higher than the 
temperature above indicated, the author drew attention to the fact, 
that the bubble of air occupied always a much smaller portion of 
the cavity than the fluid, a condition which could not obtain, if, as 
other writers hold, the fluids were enclosed under intense heat and 
pressure. 
For the purpose of accurately determining the temperatures at 
which the bubble vanished and reappeared, the author constructed 
an apparatus which he exhibited and described. It consists of a 
microscope with a hollow iron stage, having a tube in the centre to 
admit light from the reflector. At one side, and inserted into the 
stage, is a small tin retort with a stopper; at the other side, a tube 
NEW SERIES.—VOL XIV, NO. 1.—JuLy 1861. T 
