ON THE MICROSCOPICAL STRUCTURE OF ROCKS. 
3 
to show minute organisms, such as foraminifera, diatomacege, or 
faint vegetable traces in rocks in which without its help nothing 
can be detected. For the purposes of examination we may 
consider the rocks as divided into the two great classes, igneous 
and aqueous. It is often difficult to distinguish between some 
of these without the microscope. There are some fine-grained 
sandstones that to the eye might at first appear little different 
from some of the rocks of igneous origin, and vice versa ; and 
even a chemical analysis will sometimes fail to show the differ- 
ence. But subject a thin section or even splinter of such a 
rock to the microscope and the difficulty vanishes. The broken 
and often water-worn fragments of the aqueous rock, derived, it 
may be, in the first instance, from the breaking up of igneous 
rocks, will at once reveal its origin. 
The igneous rocks are those which have most engaged the 
attention of the microscopist ; the variations of structure, the 
great variety of minerals composing them, and the great inte- 
rest attached to their history, give them a pre-eminent place in 
our estimation, and this the more especially as from their waste 
and decomposition the bulk of the other rocks have been de- 
rived. 
Igneous rocks are for the most part crystalline in their struc- 
ture, although we must at the same time remember that many 
crystalline rocks, or portions of rocks, have been formed di- 
rectly from watery solution. Gypsum, calcite, rock salt, and 
some forms of quartz are examples of such. But in all such 
cases, those that have been thus formed from water may be 
readily distinguished from those which own a fiery birth, by 
means of the microscope. In his valuable paper fiC On the Mi- 
croscopic Character of some Crystals,” Mr. Sorby calls particular 
attention to certain minute cavities, almost invariably to be 
detected in even the smallest crystals, and he shows how these 
microscopic cavities are the key to the history of the crystal. 
These cavities he divides into four classes — water, gas, stone, 
and glass cavities. Water or fluid cavities may often be seen in 
quartz, and they generally contain a minute bubble, owing to 
the contraction of the liquid when cooled. When these bubbles 
are seen to move, as they frequently do, we are at once furnished 
with a proof that the cavity contains a liquid. The difference 
between full fluid cavities and those containing only air or gas 
is marked, the former being nearly invisible by reflected light, 
the latter shining brilliantly ; with transmitted light, these have 
also a very broad dark outline. A cavity containing an immov- 
able bubble or bubbles is probably a glass cavity, and the sur- 
rounding zone of a glass cavity is also wider than that of a fluid 
cavity ; the bright central spot seen with a particular adjust- 
ment of the focus is, in the case of spheroidal bubbles, relatively 
