Mr Maclaren on Ancient Beaches near Stirling, 407 



as organic substances, are liable to decomposition when exposed to the 

 elements. Mr Darwin informs us, in his Memoir on Glenroy, that he 

 had found shells on the coast of Peru, in a gradually increasing- state of 

 decay as ho ascended from the beach, till at a certain height a mere 

 layer of calcareous powder, without a vestigo of structure, alone re- 

 mained. The higher parts of the shore having been first raised above 

 the water, the shells deposited on them had suffered most from being 

 longest exposed to meteoric action. He farther states, on the authority 

 of Mr Lyell, that on the coast of Forfarshire sea-shells are found in 

 gravel-beds up to the height of 50 or 60 feet, but are wanting in simi- 

 lar beds which exist at greater elevations. In Norway, again, where 

 deposits of shingle and sand exist up to the height of 600 feet, shells 

 are found only in those which are 200 feet or less above the sea.* We 

 can thus understand how the Airthrey terrace, though a marine deposit, 

 may be destitute of shells, while they abound in the Carse, at a lower 

 level by 100 feet. Much, however, may depend on the nature of the 

 envelope. Those embedded in a clay impervious to water may re- 

 main entire for a vast length of time, while those buried in sand or 

 gravel will decay rapidly. Fuller information as to the levels at which 

 marine remains exist in Britain, may be found in Mr Murchison's 

 address to the Geological Society for 1843. 



The preservation of this remnant of the ancient beach or sea-bottom, 

 while the southern portion of it has been swept away, admits of a 

 satisfactory explanation. The sea may have subsided either by sudden 

 starts, as on the coast of Peru in 1821 ; or gradually and insensibly, 

 as on the east coast of Sweden ; or gradually, but with pauses at cer- 

 tain intervals, as on the coast of Lapland.f Let us suppose the change 

 to have been sudden. At each subsidence, the water would retire from 

 the sides of the valley towards the middle ; and after it had fallen 

 through a certain space, say 10 or 20 feet or yards, and come to a state 

 of rest, the tide, in advancing and receding, would attack those parts 

 of the ancient bottom which were above the low-water line. An open- 

 ing once made, a sea-cliff would be formed at the strand ; yard after 

 yard of the cliff would be undermined and swept away ; and if the sea 

 remained long enough at the same level, and its erosive action was not 

 stopped or turned aside by some firm resisting body, the whole of the 

 ancient beach would disappear, leaving no trace of its existence. 



Such a resisting body we have in Abbey Crag, (A in the map and 

 section 1) which is 350 feet in height, and two-thirds of a mile in 

 breadth. When the sea was many fathoms above its present level, 

 the tide setting through the channel scarcely one mile in breadth be- 

 tween Stirling Rock (S) and Abbey Crag (A), would act with great 

 force in a north-west and south-east direction^ and rapidly cut away 

 the portion of the ancient beach in front of it, but the portion behind 



* Darwin's Memoir on Glenroy, Phil. Trans., 1839. Part i., p. 63. 

 t Rapport Bur un Mcmoire de M. A. Bravais. Compter JRendus, SceanQe, 

 31st Oct. 1842. 



