Vol. 6l.~] STRUCTURE OE THE SGURR OF EIGG. 51 



by insensible gradations. Such a sheet would seem to represent a 

 portion of magma forced out from a place where solidification was 

 not yet complete and piercing a part of the mass which had already 

 consolidated as pitchstone. Trie relationship between the two rocks 

 must thus be of a peculiarly intimate kind, since they are on this 

 supposition products of one magma, intruded in the first place as a 

 single body. 



It would not follow from the relation suggested that the felsite- 

 sheets should agree exactly in chemical composition with the pitch- 

 stone (which itself seems to be somewhat variable) or with one 

 another. Indeed the crystallization of the felsite and the vitrifica- 

 tion of the pitchstone may be determined, in part, by a certain 

 difference in composition.. The following partial analyses are 

 furnished by Dr. W. Pollard, the felsite being from the conspicuous 

 sheet at the eastern base of the Sgurr : — 



Pitchstone. Felsite. 



Si0 2 63-34 6843 



Na.,0 4-56 4-76 



K 2 4-50 6-31 



Another consideration relates to the channel by which the pitch- 

 stone ascended through the basalts. We may safely assume that 

 the magma which gave birth to the rock possessed a considerable 

 degree of viscosity. As a subaerial lava it could not flow to any 

 very great distance. If it was a subterranean injection, the same 

 remark applies, though with somewhat less force. Among our 

 Tertiary intrusions, sheets of acid or subacid composition, although 

 they may equal or exceed the dolerite-siils in thickness, in no case 

 rival them in lateral extent. On either alternative, then, we might 

 hope to find some indication of the source of the pitchstone at no 

 great distance from the Sgurr itself. On the lava-flow hypothesis 

 we should seek such indication in an easterly or south-easterly 

 direction. Here we find no intrusive mass which can be supposed 

 to mark the channel of uprise ; but, since we arrive at the sea in a 

 distance of about 1| miles, the negative evidence is of little weight. 

 Close to the ridge of the Sgurr, however, on its south-western side, 

 we find a remarkable intrusion of porphyritic felsite, which may 

 possibly represent the feeder of the pitchstone-intrusion. This mass, 

 recognized by Sir Archibald Geikie and marked on his sketch-map, 1 

 has an elongated form, and extends for more than a mile close to 

 the base of the pitchstone-escarpment (see map, fig. 1, p. 44). In 

 its western or north-western part, above the deserted crofter town- 

 ships of Grulin, the outcrop has a width of 100 to 200 yards, and 

 it is evident that the rock cuts abruptly through the basalt-lavas 

 and dolerite-sills with a quasi-vertical boundary (see section, fig. 2, 

 p. 45). Eastward, near the summit of the Sgurr, the outcrop tapers 

 away, and it is possible that the mass has in this part the form of 

 an inclined sheet. The rock is not only later than the dolerite-sills, 



1 Quart. Journ. Geol. Sue. vol. xxvii (1871) p. l'8G. 



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