VARIETIES OF AGATE, ETC. 133 



amorphous or non-crystalline form of hydrated silica, containing 

 water in varying proportions of from 5 to 12 per cent. It is 

 brittle and has a conchoidal fracture. Precious opal owes its 

 distinction to a rich iridescent play of colours. Common and 

 semi-opal are opaque or semi-translucent, white or yellowish brown, 

 and are not used in jewelry. Fire opal is red and brilliant, with 

 bright reflections. 



Flint is a variety of silica allied to chalcedony, but distin- 

 guished from it by being more opaque and brittle, and by having 

 a more perfect conchoidal fracture ; also by being more capable of 

 forming sharp cutting edges, and is so hard as to scratch quartz. 

 Flint nodules are often found on the British coast, especially the 

 south coast, and the Isle of Wight, beautifully banded. They are 

 polished by lapidaries and sold as agates ; but are easily distin- 

 guished from true agates. Some red flints are passed as jaspers 

 and some as carnelians. Prof. Ruskin names flint as one of the 

 constituents of agate, but the apparent flinty portions of agate are 

 rather a variety of chalcedony than the flint of the chalk, though 

 chalcedony sometimes occurs in chalk. 



We will now consider the mode of formation of agates. It 

 has been already stated that agates occupy cavities in varieties of 

 amygdaloidal trap rocks. Several problems difficult of solution 

 here present themselves : — 



I. — How were the cavities produced? 



2. — Whence the supply of silica of which the agates are 

 composed ? 



3. — How the silica entered the cavities and became arranged 

 in bands, etc. 



4.— How was the colouring effected ? 



I. — As to the origin of the cavities.— The agate-bearing rocks 

 were originally, for the most part, molten lavas emitted from 

 ancient volcanoes. It is well known that volcanoes eject large 

 quantities of steam and gas. When these occur at the same time 

 as an eruption of lava, they fill it with steam or gas, as the case 

 may be. These bubbles rise to the surface and burst when the 

 lava is in an almost liquid condition. As it becomes more viscid, 

 the vapours or gases have less power, and as the rock or lava con- 

 solidates they escape or evaporate, leaving cavities such as may be 



