226 



SIR ARCHIBALD GEIKIE ON THE BaSIC AND [May I 894, 



be accounted for from other causes I do not lay any stress on its 

 occurrence. 



On the western declivity of the gabbro ridge, a little south of the 

 summit, the gabbros are traversed by a well-marked dyke or vein, 

 which is represented in fig. 3. 



I 



'■--* 



Fig. 3. — View offelsitie dyke cutting gahbro (west side beloiu 

 higher part of ridge, north of Druim an Eidhne, Siege). 



It is from 8 to 12 inches 

 broad, and is traceable for 

 about 28 yards. It dif- 

 fers from the broader 

 dykes both iu form and 

 in internal structure. 

 Instead of weathering 

 into a long hollow, it 

 stnnds up as a sharp rib, 

 which, from its lighter 

 colour as well as its pro- 

 minence and persistence, 

 can readily be distin- 

 guished, even at a dis- 

 tance, from the dark basic 

 rocks around it. The 

 rock comprising this dyke 

 is a dull purplish-grey 

 felsite, weathering with 

 a pale-bluish surface. It 

 is specially marked by its 

 well-defined lines of flow- 

 structure, which, as thin laminae parallel with the bounding walls, 

 cause the rock to weather into slabs. 



South of the group of dykes shown in fig. 1 (p. 222), and a little 

 west of the mass of agglomerate, a dyke of quartz-felsite may be 

 observed traversing the gabbros and demonstrating its intrusive 

 origin by the branches that diverge from it. A portion of this 

 dyke is represented in fig. 4 (p. 227). It is about 1 foot broad, and 

 the portion shown in the figure is 9 feet long. The rock con- 

 sists of felsite containing double pyramids of quartz. It gives off 

 small veins, 4 or 5 inches or less in width, which run as wavy 

 ribbons for some yards through the basic rock, and die out in mere 

 threads. Thus, while the broader dykes, like the marginal portions 

 of the main body of granophyre, show flow-structure and spherulites 

 on a great scale, these structures grow finer as the injected portions 

 of acid rock diminish in size, until they cease to be visible. 



It is not necessary to cite more examples from this locality. I 

 will only say that veins and dykes of the acid material are not con- 

 fined to the immediate proximity of the visible mass of granophyre. 

 They may be observed traversing the dark crags of gabbro even up 



