315 



The tourmaline is sometimes found to be scattered through the rock as a 

 normal constituent; at other times it is aggregated in nests and veins. 

 DE LA BECHE mentions the occurrence of a curious concretionary arrange- 

 ment in the case of a block found near Blatchford, Cornwood. In the 

 centre was an ellipsoidal cavity measuring about one foot in its longest 

 diameter. This was not quite filled with long crystals of tourmaline 

 crossing each other in different directions but with a general tendency 

 towards the centre. Surrounding this in a concentric manner was a zone 

 of tourmaline and quartz, the latter mineral predominating, and outside 

 this again was a zone of dark schorl-rock. This concretionary ellipsoidal 

 patch, measuring two feet in its largest diameter, occurred in a light 

 flesh-coloured granite. A remarkable rock composed of tourmaline and 

 quartz occurs at the Single Rose Clay Works. It contains large pseudo- 

 morphs of granular quartz after felspar in a black ground-mass of schorl- 

 rock. The rock itself may be regarded as a pseudomorph after a 

 porphyritic granite. It is somewhat remarkable that the pseudomorphs 

 after the large porphyritic felspars should be almost entirely free from 

 tourmaline. The well-known luxullianite W bears some relation to the 

 above. It consists of porphyritic crystals of pink orthoclase in Carlsbad 

 twins embedded in a black matrix of schorl-rock If the large felspars 

 in this rock were replaced by granular quartz the resulting product would 

 bear the closest resemblance rnacroscopically to the rock of Single Rose. 

 The tourmaline of luxullianite is present in two forms (1) as brown 

 crystals and grains and (2) as radiating acicular crystals. The acicular 

 crystals lie embedded in quartz. The orthoclase is turbid in thin section. 

 A consideration of the distribution of tourmaline in the granites of 

 Cornwall points to the conclusion that it must be regarded rather as a 

 secondary than an original product. Exhalations of boracic, fluoric and 

 other acids accompanying, it may be, the final stages of igneous activity 

 have evidently exerted a profound mineralizing effect not only on the 

 granites but also on the surrounding rocks. (2) Nevertheless it must be 

 admitted that in many cases the tourmaline plays the role of a normal 

 constituent ; in other words its relation to the other constituents as seen 

 in thin section is difficult to account for on the theory that it is of 

 secondary origin. 



The passage of normal granite into greisen is in some respects ana- 

 logous to the passage of the same rock into a compound of tourmaline 

 and quartz. Felspar disappears and topaz is generally developed. 

 Excellent illustrations of this passage may be seen on the south-east 

 shore of St. Michael's Mount. The principal joint planes in the granite 

 run approximately E. and W. These carry quartz, cassiterite, topaz, 

 wolfram and apatite. They are bordered on either side by a band of 

 greisen, from six inches to a foot in width, which shades off insensibly 

 into the granite. The greisen is a dark grey rock composed of quartz, 

 white mica, topaz and a little brown tourmaline. 



(1) BONNEY, M.M., 1877, pp. 215-222. 



(2) See DAUBREE, Geologie Experimentale, p. 61. 



