VOLCANIC ROCKS 25 
have probably originated from the same source, 
having issued in a molten form from a great depth 
in the earth ; but they present different appearances, 
because the one set cooled near or on the surface, and 
the other set at great depth. Both kinds of igneous 
rocks are closely alike in respect to the chemical 
elements they contain. Many lavas are quite glassy 
in structure, and such must have cooled very rapidly. 
We may secure a piece of pitch-stone as an example. 
This looks like bottle-glass, and seems to contain no 
crystals, but when a very thin slice is examined under 
the microscope some feathery crystals may be de- 
tected in the glassy base (see Plate 5,c). Other lavas 
are like masses of almond toffee, being composed of 
light-coloured minerals set like nut-kernels in a base 
of darker rock. Pumice-stone is of volcanic origin, 
and its form is due to the action of steam or gas on 
molten rock. The steam or gas made the liquid 
lava frothy, and it cooled as a spongy mass, full of 
cells. Good examples of volcanic rocks are seen 
in the famous Giant’s Causeway in Ireland, and 
Fingal’s Cave in Staffa. The lavas there in cooling 
have shrunk into magnificent six-sided columns. 
Much can be learned about ancient volcanic 
activity from the manner in which existing vol- 
canoes behave themselves. Vesuvius, Etna, and 
Stromboli, have taught us some valuable lessons, but 
as I cannot devote the whole of this book to geology, 
I must pass them by with simple mention. We 
have no active volcanoes in Great Britain now, but 
we have abundant evidence of great volcanic activity 
in past ages. You can find this evidence in massive 
areas of ancient lavas, such as I can see through my 
