154 
MELO: MICROSCOPICAL STRUCTURE OF ROCKS. 
form overlying sheets of once molten lavas, or yet again as volcanic 
dust and aslies they may have spread over certain areas, ejected from 
subaerial vents. Now it has been already observed that the unaided 
eye cannot always distinguish between an aqueous and an igneous 
rock ; there are some fine grained sedimentary rocks which to the eye 
might at first appear little different from others of igneous origin, 
and vice versa ; and even a chemical analysis will fail to show the 
difference, since many of the aqueous rocks have been directly formed 
by the disintegration of the igneous. But place a thin section or 
even a splinter of such a rock on the stage of your microscope, and 
the difficulty vanishes. The broken and often water-worn fragments 
of the aqueous rock, and occasionally traces of organic remains will 
at once reveal its nature, whilst, on the other hand, the peculiar 
characteristics of the igneous specimen will leave no doubt as to its 
origin. 
Let us now begin our observations with the rocks of undoubted 
igneous character. It is with these that microscopic investigation has 
proved of the highest value, enabling us to determine not only the 
nature of the minerals which enter into their composition, even when 
very sparingly present; but also under what circumstances they were 
brought together, the relative amounts of heat and pressure to which 
they were subjected, and whether they were amongst the original con- 
stituents of the rock under examination, or were subsequently 
introduced. 
How these questions are answered I shall endeavour to show, 
premising that before we can do anything, sections of the rock 
must in almost every instance be prepared, so as to allow of their 
investigation by transmitted light. 
In examining a thin section of an igneous rock we shall see that 
it may be either a compound of more or less definitely formed crystals 
of various minerals, or it may be what is called a glass, being amor- 
phous, but often containing included cr)^stals, or grains of non-glassy 
material. In examining such a rock section, one of the first things 
that may strike us is that almost all its crystalline elements contain 
minute cavities, some of them filled with air or other gas, as carbonic 
acid, some with liquid, whilst others are filled up with solid mineral 
