14 ° 
from which are produced rocks called silicates. The purest form of 
this silica is known as Rock-crystal, from the clear varieties of which 
our “pebble” spectacles are made. In its own natural state of 
purity it is crystalline quartz—a beautiful six-sided prism terminated 
at each end by a pyramid, or more commonly there is a pyramid at 
one end, the crystal being attached by the other end to some rock. 
Some very small. perfect double-ended specimens are in a box on 
the table, and are best seen with the hand magnifier. They are 
sifted out from earth in the neighbourhood of Buxton, and are sold 
as ‘‘Buxton diamonds.”” Other specimens of various sizes are 
before you. The largest I ever saw was at Chatsworth ; it stands 
on the floor of the statuary room, and is over three feet high. 
Many specimens of quartz are sufficiently hard to cut glass ;- in 
fact it is only exceeded in hardness by the topaz, sapphire, and 
diamond. It was the substance which first received the name 
“crystal” from the Greek crystallos—ice, from the idea that it was - 
water so far congealed that it could never be restored to the liquid 
condition. In an opaque amorphous form it is a very common 
substance on our older mountains, e.g., in Wales and the Highlands. 
When this silica is coloured by the presence of organic matter, 
iron, &c., we get the “stones” known as amethyst, cairngorm, 
chalcedony, carnelian, jasper, agate, onyx, opal, and others. Opal 
cften replaces the original matter in petrified wood. It is largely 
deposited by boiling springs and is called sinter. Most natural 
waters of all temperatures contain some silica, of which the 
diatoms make their cases. It is probably obtained by the disinte- 
gration of various rock silicates, and so finds its way into both 
water and the soil. 
The Agate is perhaps the best known of the stones we have 
mentioned, and certainly, so far as variety is regarded, it is one of 
the most beautiful. It seems to be silica deposited from water by 
successive coatings round some nucleus. The ‘“‘Moss Agate” owes 
its appearance to what are called dendritic markings, i.e. markings 
resembling a miniature tree. They are mineral markings and not 
fossil remains. Chalcedony occurs not uncommonly in flints; in 
its purest form it is milk white with a bluish tint; more frequently 
it is red or brcwn, and from the best varieties of this colour carne- 
lians are made. When banded it passes into the Agate. 
The whiter varieties of sand are nearly pure silica. The dark 
colour of our flint stones is owing, as we shall presently see to 
organic or carbonaceous matter, and is destroyed by heat, the flint 
then becoming white. 
So far we have spoken of flint or silica only in connection with 
the mineral world. 
That it has, as I said, a close connection with living things, animal 
