168 DR GEIKIE ON THE HISTORY OF VOLCANIC ACTION 



taken from the junction with the granophyre in Glenmore in the same island, parts of 

 the augite crystals are converted into granular aggregates associated with large grains 

 and patches of magnetite. The latter mineral also assumes in some of the rocks granular 

 and even globular shapes suggestive of fusion. 



The felspars, which in most of the basic rocks are usually remarkably clear and fresh, 

 show marked kaolinisation in these altered masses. Minute dusky scales of kaolin are 

 developed in them, sometimes also with the separation of minute grains of quartz. The 

 augite shows frequent alteration to hornblende, proceeding as usual from the exterior 

 inward. In some cases only an envelope of uralite appears round the augite, while in 

 others only a kernel of the original mineral is left, or the whole crystal has been changed. 

 In many cases the altered substance appears as minute needles, blades, and fibres of 

 actinolite. Occasionally, besides the green hornblende, shred-like pieces of a strongly 

 pleochroic brown hornblende make their appearance. Serpentinous and chloritic 

 substances are not infrequent. Epidote is sometimes abundant. The titaniferous iron 

 has commonly passed more or less completely into leucoxene. Here and there a dark 

 mica may be detected. 



Some of these features remind us of those which have often been described from zones 

 of contact metamorphism. They no doubt point to the long-continued action of inter- 

 stitial water, probably at a considerable temperature, and to the mutual reactions of the 

 solutions thus obtained upon the original component minerals of the dolerites and basalts. 



(3) Relation to the Gabbros. — That the granophyres of Skye, like those of Mull and 

 Rum, invade the gabbros, has been incidentally illustrated in the foregoing part of this 

 Memoir. But as the mutual relations of the two rocks in this island have been the 

 subject of frequent reference in previous writings of geologists, it is desirable to adduce 

 some further evidence from a region which has been regarded as the typical one for 

 this feature in the geological structure of the Inner Hebrides. No geological boundary 

 is more easily traced than that between the pale reddish granophyre and the dark gabbro. 

 It can be followed with the eye up a whole mountain side, and can be examined so 

 closely that again and again the observer can walk or climb for some distance with one 

 foot on each rock. That there should ever have been any doubt about the relations 

 of the two eruptive masses is possibly explicable by the very facility with which their 

 junction can be observed. Their contrasts of form and colour made their boundary 

 over crag and ridge so clear that geologists do not seem to have taken the trouble to 

 follow it out in detail. And as the pale rock undoubtedly underlies the dark, they have 

 assumed this infrapasition to mark its earlier appearance. 



I will only cite one part of the junction line. It is easily accessible, and the 

 phenomena it displays may be regarded as typical for the whole. It lies in Glen 

 Sligachan immediately to the south of the mouth of Harta Corry. The rounded eminence 

 of Meall Dearg, which rises to the south of the two Black Lochs, belongs to the granophyre, 

 while the rugged ground to the west of it lies in the gabbro. The actual contact 

 between the two rocks can be followed from the side of Harta Corry over the ridge and 



