These difficulties are assuredly most embarrassing for those who conclude that the shell-beds point to the positions where they are found having once been the sea-bottom, and especially for those who argue that its submergence and re-emergence were the result of causes operating for thousands of years. We must remember that if Sir Charles Lyell's calculations are of any value, they understate the problem, for many of the shells found in these beds are not littoral, but deep-water shells, and the land must not only have sunk to the level of the sea, but much below, to render it a fit habitat for them. We are to believe, then, that a whole continent, which was submerged for ages in this way, has nothing left to show the fact save these shreds left at different levels and always near the present coast-lines. In regard to the differing levels at which they occur, my acute friend, Mr. Darbishire, well says:—