DURING THE TERTIARY PERIOD IN THE BRITISH ISLES. 79 



deceptive resemblance to agglomerates. One of the best examples of this resemblance 

 which have come under my notice is that of the rock on which stands Dunluce Castle, on 

 the north coast of Antrim. Huge rounded blocks of a harder consistency than the rest 

 of the rock project from the surface of the cliffs, like the bombs of a true volcanic 

 agglomerate, while the matrix in which they are wrapped has decayed from around them. 

 But an examination of this matrix will soon convince the observer that it is strongly 

 amygdaloidal, and that the apparent " bombs " are only harder and less cellular portions 

 of it. The contrast between the weathering of the two parts of the rock seems to have 

 arisen from an original variety in the relative abundance of steam-cavities. Another 

 singular instance occurs at the foot of the outlier of Fionn Chro (fig. 51), in the island of 

 Rum. A conspicuous band underlying the basalts there might readily be taken for a 

 basalt-conglomerate. But in this case, also, the apparent matrix is found to be 

 amygdaloidal, and the rounded blocks are really amygdules, sometimes a foot in length, 

 filled or lined with quartz, chalcedony, &c. 



A somewhat different structure, in which, however, the appearance of volcanic breccia 

 or agglomerate due to explosion from a vent is simulated, may be alluded to here. The 

 best instance which I have observed of it occurs at the south end of Loch-na-Mna, in the 

 island of Eigg. The basalt above referred to as occurring at this locality shows on its 

 weathered surfaces a remarkable streaky structure that gives rise to prominent thin nearly 

 parallel ribs coincident with the direction of bedding. This arrangement, probably due, 

 as I have said, to the flow of the basalt while still unconsolidated, can hardly be traced 

 with the naked eye on a fresh fracture of the rock, the whole appearing as a black 

 compact basalt. On the weathered faces, the streaky layers may be observed to have 

 been broken up, and their disconnected fragments have been involved in ordinary basalt 

 wherein this flow-structure is not developed, while large blocks and irregular masses are 

 wrapped round in a more decomposing matrix. There can be no doubt that in such 

 cases we see the effects of the disruption of chilled crusts, and the entanglement of the 

 broken pieces in the still fluid lava. 



Great variety is to be found in the thickness of different sheets of lava in the plateaux. 

 Some of them are not more than 6 or 8 feet ; others reach to 80 or 100 feet, and some- 

 times, though rarely, to even greater dimensions. In Antrim, the average thickness of 

 the flows is probably from 15 to 20 feet.* In the fine coast-sections at the Giant's 

 Causeway, however, some bands may be seen far in excess of that measurement. The 

 bed that forms the Causeway, for instance, is about 60 or 70 feet thick, and seems to 

 become even thicker further east. Along the great escarpment, 700 feet high, which 

 rises from the shores of Gribon, on the west coast of Mull, there are twenty separate beds, 

 which gives an average of 35 feet for the thickness of each flow. On the great range of 

 sea-precipices, on the west coast of Skye, which present the most stupendous section of 

 the basalts anywhere to be seen within the limits of the British Islands, the average 

 thickness of the beds can be conveniently measured. At the Talisker cliffs some of the 



* See Explanation of Sheet 20, Geol. Survey, Ireland, p. 11. 



