144 DR GEIKIE ON THE HISTORY OF VOLCANIC ACTION 



eye in the form of free quartz. While in chemical composition there is the strongest 

 contrast between this series and the rocks hitherto under discussion, there are also 

 marked differences in structure and mode of occurrence. With one solitary exception 

 (Scuir of Eigg), all the masses of acid rock are intrusive — that is, they have been injected 

 beneath the surface, and therefore record for us subterranean and not superficial mani- 

 festations of volcanic action. 



The existence of rocks of this class in the midst of the basic masses has I0112; been 

 recognised. They were noticed by Jameson, who described the hills between Loch 

 Sligachan and Broadford as composed of " a compound of felspar and quartz, or what imny 

 be called a granitel, with occasional veins of pitchstone." # Macculloch gave a fuller 

 account of the same region, and classed the rocks as chiefly " syenite" and " porphyry." t 

 In Antrim, also, even in the midst of the basalt-tableland, masses of "pitchstone-porphyry," 

 " pearlstone-porphyry," and "clay-porphyry" were observed and described.! In more 

 recent years Professor Zirkel has given a brief account of the so-called " syenite and 

 porphyry" of Mull and Skye§ and the late Professor Von Lasaulx fully described the 

 trachyte of Antrim, in which he recognised the occurrence of tridymite.|| 



It is remarkable that up to the present time no connected account of the petrography 

 or of the geological relations of this interesting series of rocks has been published. Yet 

 we find in it a greater variety of petrographical characters than in any other portion of 

 the British Tertiary volcanic rocks. On the one hand, it presents us with thoroughly 

 vitreous masses, some of which in their colour, lustre, and microscopic structure remind 

 us of recent obsidians. On the other hand, it affords us coarsely crystalline compounds, 

 to which we can assign no other name than granite, and which, did we not know their 

 geological position, we might class with some of the most ancient eruptive rocks. Between 

 these two extremes abundant gradations may be found. 



In dealing with such a series of intrusive rocks, we again encounter the difficulty of 

 reaching certainty as to their relative dates of eruption, since in each case all that can 

 usually be affirmed is that the intrusive mass is younger than that into which it is 

 injected. It is quite possible that protrusions of acid rocks occurred at many intervals 

 during the accumulation of the basic masses. We have already seen that in gravels 

 near the base of the basalt-plateau of Mull, and in the agglomerates of that island as 

 well as of other districts, fragments of siliceous lavas occur. It is quite certain, therefore, 

 that at the time when the basalts of the plateaux were emitted, there existed, within 

 reach of volcanic explosions, masses of felsitic rocks, fragments from which were shot up 

 the funnels of discharge. Whether portions of these rocks were actually intruded 

 into the basalt-sheets before the building up of the plateaux was completed, or 

 whether in some cases the molten material was poured out in streams of lava at the 



* Miner alogical Travels, ii. 90. 



t Western Isles, see the descriptions of Skye, Mull, and Rum. 



% Berger, Trans. Geol. Soc, iii. (1816) p. 190. 



2 Zeitsch. Deutsch. Geol. Gesellsch., xxiii. (1871) pp. 54, 77, 84, 88. 



|| Tschermak's Min. wntf Petrog. Mittheilungen, 1878, p. 412. 



