92 DR GEIKIE ON THE HISTORY OF VOLCANIC ACTION 



beds seen at the sea-level, at the mouth of Loch Scridain, gradually sink below that level 

 as they go eastward. It is not easy to get a measurement of dip among these basalts, 

 except from a distance. If we take the inclination at only 1°, the beds which are at the 

 base of the cliff on the west, must be about 700 feet below the sea on the line of Ben 

 More, which would give a total thickness of nearly 3900 feet of bedded lava below the 

 top of that mountain. We shall not probably overestimate the thickness of the Mull 

 plateau if we put it at 3500 feet. 



The base of the volcanic series of Mull can best be seen on the south coast at Carsaig, 

 and at the foot of the precipices of Gribon. As already stated, it is there found resting 

 above Cretaceous and Jurassic rocks. The lowest beds are basalt-tuffs, of the usual dull- 

 green colour. They are in places much intermingled with sandy and gravelly sediment, 

 as if the volcanic debris had fallen into water where such sediment was in course of 

 deposition. One of the most interesting features, indeed, in this basement part of the 

 series, is the occurrence of bands of non- volcanic material which accumulated after the 

 tuffs and some of the lavas had been erupted, but before the main mass of basalts. 

 Those at Carsaig include a lenticular bed, 25 feet thick, of rolled flints, which, with 

 some associated sandy bands, lies between sheets of basalt. On the opposite side of the 

 promontory is the well-known locality of Ardtun, from which the first land-plants in the 

 volcanic series were determined. The actual base of the basalts is not there seen, being 

 covered by the sea. The " leaf-beds," with their accompanying sandstones, gravels, and 

 limestone, lie upon a sheet of basalt, which in some parts is exceedingly slaggy on the 

 top, passing down into a black compact basalt, and assuming at the base of the cliff a 

 columnar arrangement, with the prisms curved and built up endways towards each other. 

 Some of the gravels exceed 30 feet in thickness, and consist of rolled flints, bits of chalk, 

 and pieces of basalt, and of other basic igneous rocks. But some of their most interest- 

 ing ingredients are pebbles of sanidine lavas, which have been recognised in them by Mr 

 G. Cole.* No known protrusions of such lavas occur anywhere beneath or interstratified 

 with the plateau-basalts. As will be afterwards shown, all the visible acid rocks, the 

 geological relations of which can be ascertained, are of younger date than these basalts. 

 I am disposed to regard the fragments found in the Ardtun conglomerates as probably 

 derived from some of the basalt-conglomerates of the plateau, in which fragments of 

 siliceous igneous rocks do occur. Though there is no evidence that any lavas of that 

 nature were poured out at the surface before or during the emission of the basalts, the 

 contents of these fragmental volcanic accumulations prove that such lavas, already 

 consolidated, lay at some depth beneath the surface, and that fragments were torn off 

 from them during the explosions that threw out the materials of the basalt-conglomerates 

 to the surface. 



Mr Stark ie Gardner has called attention to the extraordinarily fresh condition of 

 the vegetation in some of the layers of the Ardtun section. One of the leaf-beds he has 

 found to be made up for an inch or two of a pressed mass of leaves, lying layer upon 



* Quart. Jour. Geol. Soc, xliii. (1887) p. 277. 



