376 PEOF. J. W. JTJDD OJf THE TEETIAEY AKD 



cases, where no fissures can be detected, we find the crowding of 

 cavities along hands similar to those which accompany the actual 

 cracks. The explanation which I would propose to account for 

 these interesting appearances is as follows : — When any part of a 

 crystal is subjected to abnormal strain (and that such strains, re- 

 sulting eventually in the fracture of the crystals, must be constantly 

 set up in rock-masses is evident), then, according to a well-known 

 physical principle, solvent action will be promoted along such bands 

 of strain, and cavities containing liquids will be formed. Subse- 

 quently, and perhaps partly in consequence of the formation of the 

 numerous cavities, actual rupture may take place along this band, 

 which was first a plane of strain, and which, by the solvent action, 

 has been converted into a plane of weakness in the crystal. This 

 explanation also accounts for the fact that the same band of cracks 

 may be found traversing a number of adjacent crystals in a rock. 

 Similar facts have been noticed by Yogelsang, Kalkowsky, Jullyan, 

 Phillips, and other authors (see PI. X. fig. 2). In some cases the 

 liquid- enclosures are so numerous that the supersaturated solutions 

 in them can be detected by chemical methods. If a crystal full of 

 liquid-enclosures be carefully washed and, after being crushed, 

 treated with distilled water, the liberated sulphates and chlorides 

 will give distinct precipitates with chloride of barium and nitrate of 

 silver. In many cases the enclosures are now seen to be filled up 

 with solid substances, as was pointed out to be the case in the rocks 

 of Brittany by Mr. C. Whitman Cross * (PI. X. fig. 3). 



At still greater depths, as in the rocks of the CuchuUin Hills and 

 the western extremity of Ardnamurchan, a more or less complicated 

 avanturine structure is exhibited by the felspar. Minute black rods 

 and plates are seen to be developed along one, two, three, four, or 

 even more planes within the crystal. The planes along which 

 these enclosures are formed appear to be parallel to the macropina- 

 coid, the brachypinacoid, the two prism-faces and the basal plane 

 (see PL X. figs. 4, 5, 6) : and the planes exhibiting these pecu- 

 liarities appear to be affected in the order in which we have named 

 them. 



The dimensions of these brown rods and plates, enclosed in the 

 felspar crystals, vary within very wide limits. Occasionally they 

 are sufficiently large to be seen by the naked eye ; usually they are 

 of microscopic dimensions, while they sometimes require the use of 

 the very highest powers to discriminate their forms and characters. 



In Ardnamurchan and Skye we find examples of gabbros in 

 which the felspars exhibit a brown tint, and in thin section the 

 colour of the crystals is seen to be due to the existence of nebulous 

 masses of foreign materials distributed irregularly through them. 

 The highest microscopic oT^jectives at my command only just serve 

 to partially resolve these nebulous masses into series of rods and 

 plates, arranged along certain planes within the crystal, and only 

 distinguished from those already described by their smaller dimen- 

 sions (see PI. X. fig. 7). 



* Min. und petr. Mitth. vol. iii. (18S0) p. 374. 



