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 perhaps safer to refer their formation to the agency of temporary land-floods, than to that of violent changes of level, now elevating and now depressing the surface. There occur, for instance, among the marine Oolites of Brora,--the discovery of Mr. Robertson, of Inverugie,--two strata containing fresh-water fossils in abundance; but the one stratum is little more than an inch in thickness,--the other little more than a foot; and it seems considerably more probable, that such deposits should have owed their existence to extraordinary land-floods, like those which in 1829