82 DR GE1KIE ON THE HISTORY OF VOLCANIC ACTION 



thickness. Their coarseness and repetition on successive horizons show that they 

 accumulated in the near neighbourhood of one or more small vents, from which discharges 

 of fragmentary materials took place at the beginning or at the close of an outflow of 

 lava. More commonly, however, the dirty-green or dark-brown granular matrix exceeds 

 in bulk the stones embedded in it. It has obviously been derived mainly from the 

 trituration of already cooled basalt-masses, and probably also from explosions of the still 

 molten rock in the vents. As in the case of the agglomerates of the vents, pieces of 

 older acid lavas, and still more of the non- volcanic rocks that underlie the plateaux, are 

 found in these bedded conglomerates and breccias. In Antrim and Mull, for instance, 

 fragments of flint and chalk are of common occurrence. A characteristic example of this 

 kind of rock is to be seen forming the platform of the remarkable columnar bed out of 

 which Fingal's Cave, Staffa, has been excavated. 



(/3) Felsitic Breccia and Conglomerate. — This variety is of rare occurrence, but it is 

 to be seen in a number of localities in the island of Mull. It is composed in great 

 measure of angular fragments of a close-grained flinty felsitic rock, with pieces of quartzite 

 and amygdaloidal basalt, the dull dirty-green matrix appearing to be made up chiefly of 

 basalt-dust. 



(y) Breccias of Non- Volcanic Materials. — These, the most exceptional of all the 

 fragmentary intercalations in the plateaux, consist almost wholly of angular blocks of 

 rocks which are known to underlie the basalts, but with a variable admixture of basalt 

 fragments. They are due to volcanic explosions which shattered the subjacent older crust 

 of rocks, and discharged fragments of these from the vents or allowed them to be borne 

 upwards on an ascending column of basic lava. Pieces of the non-volcanic platform are of 

 common occurrence among the fragmentary accumulations, especially in the lower parts 

 of the plateaux- basalts. But I have never seen so remarkable an example of a breccia of 

 this kind as that which occurs near the summit of Sgurr Dearg, in the east of Mull. The 

 bedded basalt encloses a lenticular band of exceedingly coarse breccia, consisting mainly 

 of angular pieces of quartzite, with fragments of amygdaloidal basalt. In the midst of 

 the breccia lies a huge mass or cake of erupted mica-schist, at least 100 yards long by 

 30 yards wide, as measured across the strike up the slope of the hill. To the west, owing 

 to the thinning out of the breccia, this piece of schist comes to lie between two beds of 

 basalt. A little higher up, other smaller but still large blocks of similar schist are involved 

 in the basalt, as shown in fig. 19. As the huge cake of mica-schist plunges into the hill, 

 its whole dimensions cannot be seen; but there are visible, at least, 15,000 cubic yards, 

 which must weigh more than 30,000 tons. Blocks of quartzite of less dimensions occur 

 in the basalts on Loch Spelve. There can be no doubt, I think, that these enormous 

 fragments were torn off from the underlying crystalline schists which form the framework 

 of the western Highlands, and were floated upward in an ascending flow of molten basalt. 

 Had the largest mass occurred at or near the base of the volcanic series, its size and 

 position would have been less remarkable. But it lies more than 2000 feet up in the 

 basalts, and hence must have been borne upward for more than that height. A similar 



