GEOLOGY wreck is such that one moment the deck is bare and the guns visible, only in a very short interval to be completely reburied beneath the gravel. But the Hydrographical Survey furnishes us with an accurate chart of the sea floor, and in many instances with the nature of the deposits which line it. We thus know that our Cornish promontory emerges from a broad submarine plain, which, gradually deepening westward, far beyond the coast of Ireland, is suddenly truncated by a great submarine steep, which plunges into the abysmal depths of the Atlantic. On the inner fringe of this submarine plain the debris of our Cornish land is being assorted. These accumulations are steadily receiving fresh accession of material and are growing at the expense of the dry land ; and we cannot escape the conclusion that the fretting back of our coasts, assisted by the denudation of the interior, must, if unchecked, eventually involve the complete removal of the county below the surface of the waves. Far distant as such an epoch must be, the interval would repre- sent a small proportion of the time that has elapsed since our rock formations were deposited. If the present rate of degradation were con- tinued for a million years, our county would in all probability be reduced to a group of islands, mainly composed of granite, which rearing their crests above the sea would still yield a dogged resistance to its ravages, as the Scilly Isles do to-day. The examination of our Cornish coast not only teaches us that the county is silently crumbling away before the insidious advances of the sea, but brings us face to face with a more mysterious factor, the past operations of which, if repeated in the future, may either turn the tide of war in favour of the land, or by acting in alliance with the sea may hasten the time of its ultimate destruction. When we see that ancientforests on the one hand fringe our coasts beneath the limits of our lowest tides ; and when on the other hand we find the remains of former beaches above the level of the highest tides, it is evident that the sea is operating on an unstable coast, subject to vertical oscillations, by which its destructive powers are controlled. The causes of these crustal movements do not immediately concern us in the present sketch, as they are the effects of subterranean agencies on which we can but speculate ; but the results of such oscillations, and the actual knowledge of the instability of the earth's crust, are concrete facts which underlie the elucidation of the complex architecture of the rocky platform which forms our county. 1 So far back as the year 1757 the submarine forest of Mounts Bay was noted by the Rev. W. Borlase, and was subsequently described by Dr. Boase in the year 1822. The latter represents it as buried beneath deposits of sand and gravel, the removal of which by the sea is constantly laying it bare the outward prolongation of the vegetable bed extending beneath the sea. Between Penzance and Newlyn he notes a bed of vegetable 1 See W. A. E. Ussher on ' The Recent Geology of Cornwall ' (articles reprinted from the Geol. Mag.), 1879 > anc * the Post-Tertiary Geology of Cornwall (printed for private circulation), 1879. 7
