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acicular fibres, but it also contains a small quantity of 
amorphous silica, perhaps opal. The fibrous structure becomes 
very apparent when a polished section is etched with hydro- 
fluoric acid. In, many instances the chalcedony passes into 
crystallised quartz, which has sometimes an amethystine colour. 
Small groups of crystallised iron, pyrites occur sparingly on the 
surface of the chalcedony in some of the veins. Between the 
first and second layers of the chalcedony a very thin crust of a 
green earth, resembling chlorophseite, is sometimes found. The 
banded arrangement of the chalcedony, layer upon layer, 
parallel to the vein-sides, is often varied by onyx-layers of 
milk-opal and wax-opal, perfectly level and apparently formed 
under the influence of gravity. A variety of cacholong, com- 
posed of opal, with an admixture of zeolitic and chalcedonic- 
quartz fibres:, is also found in the onyx-structures of the veins. 
Gault has pointed out that one kind of opal found at Cam- 
money becomes very strongly phosphorescent when it is pressed 
against a revolving grindstone. Tribe-luminescence appears to 
be a marked characteristic of both chalcedony and opal. This 
phenomena is often observed by those who are accustomed to 
polish agates and other siliceous stones. 
5- Mineralogical opinion on the mode of formation, or 
origin, of the various chalcedonies found in volcanic lavas is 
much divided, and on the whole, rather indefinite. One 
hundred years ago the early mineralogists regarded the 
chalcedony as a contemporaneous product of the lava, in which 
it is found. To-day the general opinion, expressed in well- 
known text-books on geology and mineralogy, is that the 
chalcedony found in the cavities of volcanic lavas is a 
decomposition-product of the weathering and decomposing rock. 
For example, let us take the common variety of banded chal- 
cedony known as ‘agate’ or ‘Scotch pebble.’ * In the old days 
Jameson described agate as contemporaneous with the lava in 
which it is found embedded. In more recent times Heddle, 
who studied agate very carefully, came to the conclusion that 
it was purely a decomposition product formed in the ordinary 
course of rock-weathering. This view has been stronglv upheld 
