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. 



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