Marvels of 
RHYOLITE. 
Showing fluidal structure of the diferently-coloured glassy 
bands. Magnified 15 times. 
ae WR. 
[P. Primrose. 
SPHERULITIC RHYOLITE. 
Showing the radial structure of the glassy matter. 
Magnified 15 times. 
Photos bu] 
the Universe 
crystals grew out of the originally fused mass or 
magma ; from the size of the individual crystals 
we may judge the rate of cooling of the mass, 
and even form an idea of the depth at which it 
has solidified; and subsequent chemical and 
physical changes leave their unmistakable mark 
upon the crystalline arrangement. 
It is difficult to classify rocks precisely, but 
two broad divisions commonly accepted are: (1) 
Igneous rocks, in the formation of which fire 
had an important part; and (2) Sedi- 
mentary rocks, which are made up of broken 
materials and organisms deposited by water and 
wind. The first set are often subdivided into 
groups according to their chemical composition— 
acid or basic ; their physical character—vitreous 
or crystalline ; their mode of occurrence, as deep- 
seated rocks, 
and 
has 
(plutonic) or surface 
mineral constitution and order of 
sequence in time may also have to be taken into 
The second set are often called 
they are 
(volcanic) 
their 
consideration. 
clastic or rocks, and 
grouped according as their formation has been 
brought about by mechanical, chemical or or- 
fragmental 
ganic means. 
The study of the Igneous rocks is perhaps 
the most interesting, and they can be likened 
to slags, since they are chiefly silicates which 
have solidified from the molten condition with 
varying proportions of silica (the acid portion) 
and oxides of various metals (the basic portion), 
and the ratio of these two determines whether 
the rock is said to be of an acid or a basic 
nature, or it may be intermediate, when it is 
called neutral. 
Granite is the most common of the acid rocks, 
and its structure indicates that it has been cooled 
from fusion very slowly and under great pressure, 
so that the whole mass is composed of crystals, 
of which the first to form is felspar, then mica, 
and lastly the quartz or excess silica which fills in 
Sometimes the 
cooling has been so slow, extending to several 
the interspaces left by the others. 
thousand years, that some of the crystals attain a 
length of several inches ; as in Dartmoor granite. 
They can then be examined without the aid of 
the microscope, but occasionally the quartz has 
crystallized simultaneously with the felspar in a 
very fine-grained, graphic intergrowth to which the 
