106 DR GEIKIE ON THE HISTORY OF VOLCANIC ACTION 



by later intrusions of crystalline rocks and reduced to such a fragmentary condition by 

 denudation, some interesting examples of agglomerate necks have been laid bare. The 

 largest of these occurs on the north shore at Faskadale. Cut open by the sea for more 

 than a quarter of a mile, this neck is seen to be filled with a coarse agglomerate, composed 

 mainly of basalt-blocks and debris, but crowded also with angular and subangular pieces 

 of different close-grained felsitic and porphyritic rocks belonging to the acid series to be 

 afterwards described.* Some of these stones exhibit a very perfect flow-structure, and 

 closely resemble certain fine-grained flinty intrusive rocks in Mull, to which allusion will 

 subsequently be made. The matrix of the agglomerate is of the usual dull dirty-green 

 colour, but is so intensely indurated that on a fresh fracture it can hardly be distinguished 

 from some of the crystalline rocks of the locality. The neck is pierced in all directions 

 with dykes and veins of basalt, dolerite, gabbro, and felsitic rocks. Similar intrusions 

 continue and increase in numbers further west until the cliffs become a labyrinth of dykes 

 and veins running through a mass of rock which appears to consist mainly of dull dolerites 

 and fine gabbros. Though the relations of this vent to the plateau-basalts are not quite 

 plain, the agglomerate seemed to me to rise out of these rocks. At least the basalts extend 

 from Achateny to Faskadale, but, as they are followed westwards, they are more and 

 more invaded by eruptive sheets, and assume the indurated character to which I have 

 already referred. 



On the south side of the peninsula of Ardnamurchan, another neck, noticed by 

 Professor Judd, rises into the bold headland of Maclean's Nose, at the mouth of Loch 

 Sunart, and affords better evidence of its relation to the bedded basalts. It measures 

 about 1000 yards in length by 300 in breadth, and its summit rises more than 900 feet 

 above the sea, which washes the base of its southern front. It is filled with an agglo- 

 merate even coarser than that on the northern coast. The blocks are of all sizes, up to 

 eight or ten feet in diameter. By far the largest proportion of them consists of varieties 

 of basalt, slaggy and vesicular structures being especially conspicuous. There are also 

 large blocks of different porphyries and felsitic rocks like those just referred to, a 

 porphyry with felspar crystals two inches long being particularly abundant. All the 

 stones are more or less rounded, and are wrapped up in a dull-green compact matrix of 

 basalt-debris. There is no stratification or structure of any kind in the mass. Numerous 

 dykes or veins, some of basalt, others of a porphyry, resembling that of Craignure, in 

 Mull, traverse the agglomerate. 



The position of this vent, with reference to the surrounding rocks, will be best 

 understood from the map (Plate L), and from the subjoined section (fig. 30). On the 



* One of these felsites when viewed under a high magnifying power is seen to present an abundant development of 

 exceedingly minute micropegmatite arranged in patches and streaks parallel with the lines of fluxion structure in the 

 general cryptocrystalline ground mass. The close relationship between the felsites, quartz-porphyries, and grano- 

 phyres will be afterwards pointed out in the description of the acid rocks.' It is remarkable that, though these rock* 

 occur abundantly in fragments in the volcanic necks and agglomerates of the plateaux, not a single instance has been 

 observed of their intercalation as contemporaneous sheets among the basic lavas. An analogous case of the interstratifi- 

 cation of felsitic tuffs among basic lavas occurs in the volcanic series of the Old Red Sandstone of central Scotland. 



