DYKES AND SILLS 27 
some say, much more rapidly than the harder vol- 
canic material of the dykes, and so, after ages of 
wear and tear, the dykes appear like rugged walls. 
In other cases the molten lava has hardened sand- 
stone by its intense heat—turning it into quartzite— 
and then the material of the dyke is often softer than 
the rock in its close neighbourhood. In consequence 
of this the dyke weathers quicker than the rock 
through which it runs, and the dyke is evident 
principally by the mould in which it was formed. 
Reference to illustration on Plate 6, 6, will help you 
to understand what I am describing far better than 
many words of explanation. Dykes, it should be 
mentioned, are always vertical, or nearly so. Sills © 
are sheets of volcanic material which has not made 
its way to the surface, but has forced itself between 
layers of rocks and there solidified. Such sills are 
sometimes discovered in districts of past volcanic 
activity when railway cuttings are made, or stone 
is being quarried. Sills are not upright like dykes, 
but horizontal or inclining thereto. 
The igneous rocks are also spoken of as Primary. 
They were first in the field, so to speak, and they 
have provided the material from which the other 
rocks, called Stratified or Secondary, have been 
formed. 
The StratirieD Rocks, I find, interest young 
observers much more intensely than the igneous, and 
they may well do so, seeing that they contain fossils. 
There must, indeed, be something very far wrong 
with the boy or girl who is not curious about a 
fossil, for is it not a record in stone of some creature, 
or plant, that existed perhaps millions of years ago ? 
