occasional protrusions
and land-slips, we find some difficulty in tracing their order of
succession.

Near the base of the slope, where the soil has been undermined and the
rock laid bare by the waves, there occur beds of a bituminous black
shale,--resembling the dark shales so common in the Coal Measures,--that
seem to be of fresh water or estuary origin. Their fossils, though
numerous, are ill preserved; but we detect in them scales and plates of
fishes, at least two species of minute bivalves, one of which very much
resembles a Cyclas; and in some of the fragments, shells of Cypris lie
embedded in considerable abundance. After all that has been said and
written by way of accounting for those alternations of lacustrine with
marine remains, which are of such frequent occurrence in the various
formations, secondary and tertiary, from the Coal Measures downwards, it
does seem strange enough that the estuary, or fresh-water lake, should
so often in the old geologic periods have changed places with the sea.
It is comparatively easy to conceive that the inner Hebrides should have
once existed as a broad ocean sound, bounded on one or either side by
Oolitic islands, from which streams descended, sweeping with them, to
the marine depths, productions, animal and vegetable, of the land. But
it is less easy to conceive, that in that sound, the area covered by the
ocean one year should have been covered by a fresh-water lake in perhaps
the next, and then by the ocean again a few years after. And yet among
the Oolitic deposits of the Hebrides evidence seems to exist that
changes of this nature actually took place. I am not inclined to found
much on the apparently fresh-water character of the bituminous shales of
Eigg;--the embedded fossils are all too obscure to be admitted in
evidence; but there can exist no doubt that fresh water, or at least
estuary formations, do occur among the marine Oolites of the Hebrides.
Sir R. Murchison, one of the most cautious, as he is certainly one of
the most distinguished, of living geologists, found in a northern
district of Skye, in 1826, a deposit containing Cyclas, Paludina,
Neritina,--all shells of unequivocally fresh-water origin,--which must
have been formed, he concludes, in either a lake or estuary. What had
been sea at one period had been estuary or lake at another. In every
case, however, in which these intercalated deposits are restricted to
single strata of no great thickness, it is p