372 PEOF. J. W. JUBT) 01^ THE PEOrTLITES 



by the fact that they are found intersecting the basaltic sheets as 

 veins or dykes, while at other times they can be shown to lie upon 

 their greatly eroded surfaces, as lava-currents. Of the same late age 

 are certain other rocks, some of more basic and others of more acid 

 composition, w^hich will be considered in greater detail hereafter. 

 The -whole of these rocks belong to the latest of the three periods to 

 which, as I showed in 1874, the Tertiary volcanic rocks of the 

 Western Islands must be assigned. 



The lavas in question are of very considerable interest as having 

 been undoubtedly the latest-erupted volcanic masses in the British 

 Islands. A careful study of them shows that they present the most 

 striking resemblances to some of the recent volcanic rocks of 

 Iceland. Dykes and veins of these lavas are found traversing the 

 thick ophitic olivine-basalts of the Western Isles of Scotland and of 

 Antrim. But other dykes of remarkably similar rock occur cutting 

 through the Palaeozoic rocks of the lowland districts of Scotland, 

 and these reappear in the north of England, where some of them 

 can be shown to intersect Jurassic strata. Some of the Scottish 

 examples of these rocks have been well described by Dr. A. 

 Geikie*. The English examples have been admirably studied by 

 Mr. Teallt. 



I have already pointed out J that along these lines of fissure now 

 occupied by dykes there is evidence of the outburst of a volcanic 

 material giving rise to lines of volcanic cones, which bore the same 

 relation to the great volcanoes of the Western Isles, that the chains 

 of " puys " in Auvergne did to the great volcanic mountains of 

 Mont Dore, the Mezen, and the Cantal. 



At two points only, so far as I know, have the lava-currents and 

 tuffs of this period been preserved. This is accounted for by the fact 

 that the amount of denudation in the district since the formation of 

 these small, subsidiary volcanic cones has been excessive ; and only 

 where the lava-currents were of unusual dimensions, or were of such 

 a character as to resist the action of denuding agencies in an 

 exceptional manner, was there any chance of their being preserved 

 for our study at the present day. 



The first case of the kind noticed Avas that of the Sgurr of Eigg, 

 which was so well described by Dr. A. Geikie in 1871 §. His 

 explanation of the mode of preservation of several successive lava- 

 sheets, by their being poured out into a valley that had been eroded 

 in the basaltic plateau, is one that must commend itself to every one 

 who has studied the district. Equally convincing is the evidence 

 he adduces of the enormous amount of denudation that has taken 

 place since the formation of these lava-flows, seeing that the basalts 

 forming the sides of the valley have all been removed, leaving the 

 later lavas as a mass crowning the summit of a long ridge. 



* Proc. Roy. Vhjs. Edinb. vol. v. (1878-80) pp. 219-254; and Trans. Rov. 

 Soc. Edinb. vol. xxxv. (1889) pp. 24-73. 

 t Q. J. a. S. vol. xl. (1884) p. 209. 

 J Q. J. a. S. vol. XXX. (1874) pp. 200-272. 

 § Quart. Journ. Geol. Soc. vol. xxvii. (1871) pp. 303-309. 



