178 DR'GEIKIE ON THE HISTORY OF VOLCANIC ACTION 



had supplied the granitoid, granophyric, porphyritic, and felsitic varieties of earlier 

 protrusions. We must remember that the pitchstone- veins are not mere local glassy parts 

 of the larger bodies of granophyre or felsite in or near which they lie. Their margins are 

 sharply defined ; they are indeed in all respects as manifestly intruded, and therefore 

 later masses, as are the basalt-dykes. Their occurrence, therefore, within the granophyre 

 bosses proves them to be younger than the youngest of the large erupted masses of the 

 Tertiary volcanic series. Whether they are also later than the latest basalt-dykes cannot 

 yet be decided, for I have never succeeded in finding an example of the intersection of 

 these two groups of veins and dykes. But, with this possible exception, the pitchstones 

 are the most recent of all the eruptive rocks of Britain. This fact acquires additional 

 interest when taken in connection with the history of the Scuir of Eigg, to be imme- 

 diately referred to. 



As a rule, the intrusive pitchstones occur as veins which cannot be traced far, and 

 which vary from a few yards to less than an inch in width. They generally show con- 

 siderable irregularity in breadth and direction, sometimes sending out strings into the 

 surrounding rock (fig. 61). The outer portions are not infrequently more glassy and 

 obsidian-like than the interior. Occasionally the vitreous character disappears by 

 devitrification, and the rock assumes the texture of a compact felsite or of a spherulite- 

 rock.* 



4. Superficial Lava Streams. 



The question whether any of the acid volcanic rocks were actually emitted at the 

 surface, fortunately does not rest in the uncertainty in which we are compelled to leave 

 the same question when asked of the bosses of gabbro, but admits of a positive answer. 

 One solitary example remains of a true superficial stream of acid lava — that of the Scuir 

 of Eigg. But when we consider the form of the granophyre cones, as already described, 

 the indication of an internal structure in some of them, corresponding more or less with 

 their external form, and the occasional presence of brecciated portions along their 

 maroins, it is difficult to believe that no connection was established between these cones 

 and the outer air. Although no " lava," using that word in Von Bugh's comprehensive 

 sense of anything that flows from a volcanic orifice, may have spread outwards from the 

 necks of granophyre, the molten material may have been protruded to the daylight in 

 some such form as the domite puys of Auvergne. A comparatively small amount of 

 denudation would suffice to efface any evidence that the igneous rock had ever reached 

 the surface. But, in actual fact, the denudation has been enormous. Not a single 

 vestige now remains of a superficial discharge from any one of the numerous granophyre 

 cones of Skye. 



The solitary remnant of an acid lava-stream, forming the conspicuous Scuir of Eigg, 

 has already been fully described by me, and for its structure and history I must refer to 

 a former paper.t contenting myself here with a brief summary, and with a statement of 



* For an account of the pitchstone veins of Eigg, see Quart. Jour. Geol. Soc, xxvii. p. 299. 

 t Qucvrt. Jour. Geol. Soc, xxvii. (1871) p. 303. 



