1056 Marvels of the Universe 
deep yellowish-brown spherulites are closely packed and some of them have been compressed into 
ovoid shapes. 
Where the lava in its flow traverses any colder solid material, it is subjected to a fairly rapid 
cooling which leaves it almost completely void of crystalline structure. In this vitreous condition 
it is called Obsidian, and even when black it possesses quite a glassy appearance. It frequently 
contains small incipient crystals or microlites of felspar arranged in the direction of the lava flow, 
and these may serve as nuclei around which spherulites of dark reddish-brown glass have formed. 
This structure is shown in a remarkable degree by some of the Obsidian found in Iceland, and when 
this rock is rendered frothy by steam 
it is called pumice. 
Should the lava encounter water, 
it is not only very rapidly cooled, 
but it is slghtly altered in structure 
so that it loses its vitreous lustre and 
becomes wax-like, when it is called 
Pitchstone. This peculiar rock is found 
in greatest abundance along the west 
coast of Scotland, and the variety 
from Corriegills on the Island of Arran 
is world-famous because of the very 
beautiful arrangement of the embryo 
crystals throughout the ground-mass 
of greenish-coloured _ glass. These 
microlites of hornblende assume the 
shape of plumes and fronds of ferns, 
so delicate in tracery that it requires 
a high magnification to reveal their 
full beauty. The separation of these 
microlites indicates that the rock con- 
taining them has the glass slightly 
devitrified, and it therefore occurs at 
a slight distance from the full chilling 
effect of the water. 
The Sedimentary rocks are gener- 
ally of more use for industrial pur- 
poses than the Igneous rocks, but 
Ve? 
Photo by] 2 ’ ; [P. Primrose. s. aire ‘ Sl e E : 
CORN TINESTONE! their structure is much less regular. 
From St. Monans, Perthshire, showing sections of coral skeletons, and the 
septa of the polyps filled with crystalline calcite. Magnified 45 times. compacted aggregates of sand grains, 
Thus the sandstones are more or less 
and the slates have been formed from clay particles. Of greater interest microscopically are 
the limestones, which may be of chemical or organic origin. In the latter case the rocks are 
composed of innumerable shells and skeletons of minute water animals, usually marine, and of a 
very low order in the scale of life. Although little more than shapeless masses of jelly, they have 
the power of secreting calcium salts from the water in which they live gregariously, and of con- 
verting these into carbonate of lime to form their shells, etc., which in time accumulate as masses 
of limestone. In some of our limestone rocks and their soft equivalent, the chalk beds, shells of 
mollusca are found which are large enough to handle, but the more delicate protozoa called 
Foraminifera require the microscope to identify them. These shells constitute the Globigerinan 
ooze, and the animals themselves are considered to be the earliest form of life which existed on the 
