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from which are produced rocks called silicates. The purest form of 

 this silica is known as Eock-crtjStal, 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 beiog 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 

 siibstance 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 

 often 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 sihca only in connection with 

 the mineral world. 



That it has, as I said, a close connection with living things, animal 



