A sheet of basalt 40 feet 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 Equisetum grew, of which the remains 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 neighbourhood 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. (Quarterly Geological Journal 1851 page 90.)
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 (Figure 153), also Corylus grossedentata, a Lower Miocene
species of Switzerland and of Menat in Auvergne.
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