DURING THE TERTIARY PERIOD IN THE BRITISH ISLES. 175 



sills in Skye proceeded. Possibly they may be concealed underneath some of the larger 

 areas of the rock, such as those of Cam Dearg or Beinn a' Chairn. In Raasay, however, 

 it is possible to connect the sheets with the funnels through which they ascended. This 

 is well seen near the Manse, where the accompanying section may be observed (fig. 59). 

 Owing to great denudation, the massive sheet of granophyre has been cut into isolated 

 outliers which cap the low hills, and the rock may be seen descending through the 

 Jurassic sandstones, which in places are much indurated. It is observable that the 

 amount of contact metamorphism induced by the granophyre sills upon the rocks between 

 which they have been injected is comparatively trifling. It is for the most part a mere 

 induration, sometimes accompanied with distortion and fracture. 



3. Veins and Dykes. 



Besides bosses and sills, the acid rocks of the Inner Hebrides take the form of veins 

 and dykes which have invaded the other members of the volcanic series. Some of these 

 have already been referred to ; but a more particular description of the venous develop- 

 ment of the acid rocks as a whole is now required. 



Considered as a petrographical group, these veins and dykes are marked by the 

 following characters. At the one extreme we have thoroughly vitreous rocks in the 

 pitchstones. Among these, however, various degrees of devitrification appear, leading us 

 to the completely devitrified felsites, which occur almost entirely as veins or dykes, 

 Occasionally the microgranitic structure makes its appearance. More frequently, how- 

 ever, the veins and dykes consist of what macroscopically are quartz-porphyries, and 

 which under the microscope can generally be resolved into granophyric porphyries or true 

 granophyres. In a few instances, the veins proceeding from the granitic bosses show a 

 granitic structure. 



In their mode of occurrence, the smaller protrusions of acid rocks differ considerably 

 from the ordinary type of the dykes and veins of the basic group. They comparatively 

 seldom form true dykes. Most frequently they occur as irregular veins, which vary much 

 within a short space in thickness and direction. They never exhibit the persistent trend 

 and parallelism of opposite walls, so distinctive of the basic dykes. The phenomena 

 attending their eruption must have been correspondingly different. Their advent -was 

 not preceded by the rending of the terrestrial crust into long parallel fissures. On the 

 contrary, they seem to have been forced between the irregular rents, joints, and bedding- 

 planes of the rocks, so that they often pursue a singularly sinuous course. 



Round the margin of the larger granophyre bosses, veins have sometimes been given 

 off in great numbers, as has been stated above. But, for considerable distances, not a 

 single vein or dyke may appear. Along the well-exposed boundary of Beinn-an- 

 Dubhaich, for example, though the edge of the granophyre is remarkably notched, there 

 are hardly any protrusions that deserve the name of veins. In the central mountainous 

 tract of Mull, veins of various porphyries and felsitic rocks are extraordinarily abundant. 



