GEOLOGY That great platform, stretching seawards and presenting to the Atlantic solid ramparts of lofty cliff, appears to withstand the constant assaults of the sea without sustaining any damage. But in spite of the obduracy of these bold headlands we know, from the effects of wave- action on softer strata, that, though imperceptibly, they must be slowly losing ground. It has been calculated that a single roller of the Atlantic ground swell (20 feet high) falls with a pressure of about a ton on every square foot ; but the winter breakers often exert a pressure of over three tons to the square foot, so that the cumulative momentum that our coast is called upon to withstand is indeed enormous. If the projecting headlands reveal small traces of the ravages made upon them by the sea, the bays, notwithstanding their more sheltered situation, readily betray their losses. While the former plunge steeply beneath the water, the softer rocks which form the bays are lined by a succession of beaches, from the platforms of which we can study the effects of wave action. It is evident from the most cursory examination of the cliff-foot that our seaboard is undergoing a slow but constant modification. The debris from the cliff which accumulates at its base is shifted by tide action, and in times of storm is hurled by the sea against the rock face from which it was detached. While this process rounds the fragments, and finally reduces them to the condition of gravel and sand, the cliff itself is gradually being undermined by the incessant pounding, and furnishes a further supply of debris, which in its turn, brought within the action of the waves, occasions a repetition of the process, and the sand and shingle so produced are still further assorted and spread on the sea floor by the action of tide and current. Not only is the cliff eroded along its base, but blocks of rock are frequently de- tached by the loosening of their joints due to the disintegrating action of the weather, such as rain and frost. This debris, if permitted to remain, would act as a shield against the continued waste of the cliff, but the moment it comes within range of the waves it is utilized as an instrument of further destruc- tion. That the gradual fretting back of our shores is a fact to be reckoned with is apparent to dwellers on the coast. During the last quarter of a century within the experience of the author, the foot- paths skirting the shores of Gerran's Bay have been gradually removed inland by the landslips along the edge of the cliff consequent on the undermining of its base ; and at the present time rents are still visible along the edge, the certain precursors of future landslips ; while large slices of the cliff are still to be seen which have not yet subsided to its base. The picturesque caverns which are so common along our sea- board are in themselves testimonies to the degradation of our coasts ; lines of weakness in the rock having, through the action of the waves, been enlarged to a wide opening. The mutual relation of our bays and headlands to the character of the rock formations is nowhere better expressed than in the termina- tion of the great headlands which enclose Mounts Bay. The bay is 3
