212 
THE TERTIARY VOLCANOES 
BOOK VIII 
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 
structure, 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 other basic igneous rocks. But some of their 
most interesting ingredients are pebbles of sanidine lavas, which have been 
recognized in them by Prof. G. Cole. 1 No known protrusions of such lavas 
occur anywhere beneath or interstratified with the plateau-basalts of this 
district. As will be afterwards shown, all the visible acid rocks, the 
geological relations of which can be ascertained, are here 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-con- 
glomerates of the plateau, in which fragments of siliceous igneous rocks do 
occur. Though there is no evidence that any lavas of that nature were here 
poured out at the surface before or during the emission of the basalts, the 
contents of these fragmental volcanic accumulations suggest that such lavas, 
already consolidated, lay at some depth beneath the surface, and that frag- 
ments were torn off from them during the explosions that threw out the 
materials of the basalt-conglomerates to the surface. 
The succession of strata at the Ardtun headland varies considerably in 
a short distance, some of the sedimentary deposits rapidly increasing or 
diminishing in thickness. The section as measured by Mr. Starkie Gardner 
is as follows 2 : — 
Columnar basalt, 40 feet. 
Position of first leaf-bed, obscured by grass, about 2 feet. 
Gravel varying from about 25 feet to a maximum of nearly 40 feet. 
Black or second leaf-bed, 2^ feet. 
Gravel about 7 feet. 
Grey clay, 2 feet. 
Laminated sandstone, 6 inches, with 3 inches of fine limestone, containing 
leaves at the base. 
Clay, with leaves at base, 1 foot. 
Cluncli, with rootlets, 7 inches. 
Amorphous basalt, becoming colmnar at base, about 60 feet. 
Mr. Starkie Gardner has called attention to the extraordinarily fresh con- 
dition of the vegetation in some of the layers of the Ardtun section. One 
of the leaf-beds lie has found to be made up for an inch or two of a pressed 
mass of leaves, lying layer upon layer, and retaining almost the colours of 
dead vegetation. Among the plants represented is a large purple Ginkgo 
and a fine Platanites, one leaf measuring 15|- inches long by 10^- broad. 
Quart. Jour. Geol. Soc. xliii. (1887) p. 277. 
2 Op. cit. p. 280. 
