Great Terrace of Erosion in Scotland. 105 



which is filled to a considerable height with terraces of detri- 

 tus, and which stretches back to the lofty mountains forming 

 the centre of the island. On an examination of these detrital 

 masses, he finds the lower part composed of a bed of blue 

 clayey drift, with small half-worn boulders scattered equally 

 through it ; over this, a bed of coarse gravel, and above this 

 again, a deep bed of fine sand. The stuff which he had seen 

 on the face of the ancient sea-cliff, is the same with the first 

 of these deposits. It consequently becomes evident that the 

 three sets of conditions which gave rise in succession to the 

 bed of blue clayey drift with boulders, the coarse gravel, and 

 the deep bed of fine sand, are all posterior to that incising 

 action of the sea which formed the terrace and sea-cliff. I 

 presume it will be readily admitted that the two higher beds 

 argue a period of submergence ; and as the surface of the whole 

 is not less than 140 feet above the present sea-level, the sub- 

 mergence must have been to that depth at least. If I am 

 right in considering the blue clayey drift as the product of a 

 glacier which once filled Glen Iorsa, then we had previously 

 had a period of low temperature in Arran. Thus, we may 

 speculate on a glacial period, and a period of deep re-immer- 

 sion, as following in succession upon the period of this terrace 

 of erosion. Nor is this the whole series of events ; for the two 

 superficial deposits argue each a separate set of conditions, 

 the coarse gravel marking a time when the embouchure of the 

 river was little way of the valley, and the bed of fine sand a 

 time when it was much farther up, and when the sea at this 

 place was of course considerably deeper. It is scarcely neces- 

 sary to remark that the time required for this succession of 

 events must have been very great. 



The arrangement of events here speculated upon has sup- 

 port in certain observations of a similar kind which have been 

 indicated by other geologists in Scotland. Mr Milne Home 

 found the deep ravine of the Water of Leith, at the Dean near 

 Edinburgh, overlaid by what he considered as a third drift 

 bed connected with erratic boulders, and argued of course that 

 the cutting of that ravine by the river had been followed by a 

 period of deep reimmersion. Mr Charles Maclaren made a 

 similar remark regarding the ravine of the river Allan between 

 Dumblane and Stirling ; but for this I have unfortunately 



