OLDER PERIDOTITES OF SCOTLAND. 379 



rounded by a shell of diallage (see PL XI. fig. 3) ; in other cases 

 the alteration of the augite into diallage is seen to take place 

 along cracks, due to cleavage or other causes, -which intersect the 

 crystal (see PI. XI. fig. 5) ; in other cases, again, the alteration 

 into diallage is found to occur in irregular patches within the 

 augite, though the cause of the distribution of these altered patches 

 may not be manifest from a study of the thin section. 



Although the alteration of the augite may be set up along the 

 cleavage-cracks of the crystals, j^et the position of the brown 

 enclosures bears no relation to the principal (prismatic) planes of 

 cleavage in the mineral. On the contrary, the enclosures appear to 

 be developed in planes parallel to the orthopinacoid planes, in which 

 only a very imperfect cleavage exists in augite *. 



In the island of Rum, the augite, though exhibiting the first traces 

 of the development of the structure which is characteristic of diallage, 

 is seldom so far altered as to deserve being called by that name. 

 In the larger igneous masses of Mull, the augite in all the central 

 portions is in the condition of diallage, as was pointed out by Zirkel. 

 Between rocks in which the augite is entirely unaltered, and those 

 in which it is completely transformed into diallage, every possible 

 transition may be found. 



In the central portions of the largest intrusive masses, those of 

 Skye and Ardnamurchan, the augite exhibits a still further modifi- 

 cation. In addition to the enclosures along the planes parallel with 

 the orthopinacoid, other enclosures make their appearance in planes 

 cutting these at an angle of 87|°, or parallel with the clinopinacoid. 

 The ordinary sections in which these two sets of enclosures are seen 

 intersecting one another at difi'erent angles, according to the 

 direction in which the sections traverse the crystals, present a 

 singular " cross-hatched" appearance ; but it is easy to trace every 

 gradation from the variety with enclosures developed along one set 

 of planes, to that in which they appear along two sets of planes. 

 Frequently another set of enclosures may be detected as making their 

 appearance along a third set of planes, which appear to be parallel 

 to the basal plane t (see PI. XI. figs. 4 & 6). 



It is especially noteworthy that the colour, lustre, and general 



* The curious augite of the Wbiu Sill, described by Mr. Teall (Quart. Journ. 

 G-eol. Soc. vol. xl. (1884) pp. 647-650) as presenting a foliated structure parallel 

 to the basal plane, not improbably owes its peculiarity, as suggested by Prof. 

 Eosenbusch, to an intergrowth of different minerals, or possibly to lamellar 

 twinning on those planes of ultra-microscopical dimensions. (SeQ also Vom 

 Rath, Zeitschr. fiir Krystall. &c., vol. v. (1881) p. 495.) Prof. Eosenbusch 

 is inclined to regard the foliation of diallage as connected with the existence of 

 lamellar twinning parallel to the orthopinacoid (Mikroskopische Physiographie, 

 vol. i. p. 303), a view which does not appear to be shared by most other petro- 

 graphers. The enclosures in planes parallel to the orthopinacoid in augite, 

 though the first formed, usually exhibit a tendency to indefiniteness and irregu 

 larity not seen in those pai'allel to the clinopinacoid and the basal plane. 



t Tschermak has pointed out that the foliation-planes in diallage sometimes 

 deviate by as much as 15*^ from the true orthopinacoid, and suggests that this 

 may be the result of pressure. A similar but smaller divei'gence from sym- 

 metrical development of the foliation planes is said to occur in hypersthene. 



