248 
ELEMENTS OE GEOLOGY. 
thick covers the"whole; and another columnar bed of the 
same rock, ten feet thick, is exposed at the bottom of the cliff. 
One of the leaf-beds consists of a compressed mass of leaves 
unaccompanied by any stems, as if they had been blown into 
a marsh where a species of Eq%metum grew, of which the re¬ 
mains are plentifully imbedded in clay. 
It is supposed by the Duke of Argyll that this formation 
was accumulated in a shallow lake or marsh in the neighbor¬ 
hood of a volcano, which emitted showers of ashes and 
streams of lava. The tufaceous envelope of the fossils may 
have fallen into the lake from the air as volcanic dust, or 
have been washed down into it as mud from the adjoining 
land. Even without the aid of organic remains we might 
have decided that the deposit was newer than the chalk, for 
chalk-flints containing cretaceous fossils were detected by 
the duke in the principal mass of volcanic ashes or tuff.* 
The late Edward Forbes observed that some of the plants 
of this formation resembled those of Croatia, described by 
Unger, and his opinion has been confirmed by Professor 
Heer, who found that the conifer most prevalent was the 
Sequoia Langsdorfii (Fig. 153, p. 238), also Corylus grosse- 
dentata^ a Lower Miocene species of Switzerland and of 
Menat in Auvergne. There is likewise a plane-tree, the 
leaves of which seem to agree with those of Platanus ace- 
raides (Fig. 141, p. 221), and a fern which is as yet peculiar 
to Mull, Fincites hebridica^ Forbes. 
These interesting discoveries in Mull led geologists to sus¬ 
pect that the basalt of Antrim, in Ireland, and of the cele¬ 
brated Giant’s Causeway, might be of the same age. The 
volcanic rocks that overlie the chalk, and some of the strata 
associated with and interstratified between masses of basalt, 
contain leaves of dicotyledonous plants, somewhat imperfect, 
but resembling the beech, oak, and plane, and also some co- 
niferse of the genera pine and Sequoia. The general dearth 
of strata in the British Isles, intermediate in age between the 
formation of the Eocene and Pliocene periods, may arise, says 
Professor Forbes, from the extent of dry land which pre¬ 
vailed in that vast interval of time. If land predominated, 
the only monuments we are likely ever to And of Miocene 
date are those of lacustrine and volcanic origin, such as the 
Bovey Coal in Devonshire, the Ardtun beds in Mull, or the 
lignites and associated basalts in Antrim. 
Lower Miocene,United States: Nebraska. —In the territory 
of Nebraska, on the Upper Missouri, near the Platte River, 
lat. 42° N., a tertiary formation occurs, consisting of white 
♦ Quart. Geol. Journal, 1851, p. 90. 
