On the Occurrence of Beds in the West of Scotland beneath the Boulder-clay

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Abstract
A n examination of the Drift-beds in the island of Arran, undertaken last summer with the view of determining their true sequence, led me to the conclusion that the lowest bed there—the lower Till or true Boulder-clay—did not contain any fossils. Having noticed, while conducting the inquiry, how many difficulties and causes of error are met with, which an observer does not encounter in the case of rocky strata, I began to doubt the accuracy of the statements so often put forward that fossils occur in this bed, and to consider that the supposed cases which are on record might be explained in various ways. Other drifts with boulders, though superior in position, may readily be mistaken for the Boulder-clay. The upper surface of this last-named bed undulates violently; and shells in part of the overlying Arctic-shell bed, occupying a hollow in the undulation, might seem to be in the older deposit, especially if, as in Arran, the laminated clay, which usually in Clydesdale divides the Boulder-clay from the shell-bed, happened to be absent,—the observer being, of course, supposed to overlook the undulation. And, further, the wash or facing, formed over the whole surface of a drift-section by atmospheric causes and runlets of water, might contain shelly fragments from the bed above, which, adhering to the surface merely, would yet appear to be in the Boulder-clay. There was yet another explanation. It was possible that fossils said to be deep in the Boulder-clay might really be in beds beneath it, of

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