1905-1906.] 343 



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. Tribo-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 strongly upheld 



