IN THE SOUTH OF ARRAN. 541 



1100 or 1200 feet at least. The striated surfaces (indicative of ice) at the pre- 

 sent sea-level prove that the land, during the boulder-clay period, stood at least 

 no lower than it does now, and the presence of the boulder- clay — a sea deposit as 

 we have seen — proves that to a height of 1100 or 1200 feet the sea rose over the 

 land. There is indeed some evidence that during the glacial period the land 

 stood higher than now, and still better proof exists, in Wales especially, that 

 the depression extended to not less than 1400 feet ; but I confine myself now to 

 the evidence afforded by the Arran beds. 



8. The subsidence was probably continuous not oscillatory. Of course there 

 may have been oscillations here, as there have been, in recent times, at Naples 

 and elsewhere ; but, so far as I know, no evidence exists of such oscillations 

 occurring during a protracted and long-sustained process of subsidence and 

 upheaval : and certainly no trace of such irregularity has yet been found in 

 connection with the movements of our country during the glacial period. At 

 the same time it is not very obvious by what evidence such oscillations, during 

 the deposition of the boulder-clay, could be established. Still, in the absence of 

 any strictly analogical case, we may consider that the subsidence of the land 

 here was continuous. 



9. The subsidence was gradual. As this opinion is opposed to that which I 

 held when I read this paper to the Society, it is right that I state somewhat fully 

 the grounds on which it rests. 



The depression of the land might take place by a succession of sudden jerks 

 or leaps, with long intervals of rest, or by a slow, steady subsidence, as is the 

 case at present in Greenland.* 



In the former case, the ice-cake would be floated off the land as it sank, and 

 between the top of the boulder-clay bank on which it had been resting, and the 

 new ice-foot, there would be a zone of bare rock perpendicularly equal to the 

 amount of the subsidence, but which, in the flatter districts, would be of con- 

 siderable extent horizontally. Along the upper edge of this zone, at its junction 

 with the ice, the boulder-clay bank would again begin to form, but all its lower 

 expanse would lie too remote from the supply of debris to be thus covered. The 

 glacier streams, however, floating out to sea on the surface of the salt water, 

 would gradually drop on it the detritus with which they are always charged, 

 and a bed of stratified sand and clay by degrees would overspread the bare rock, 

 till the growing mass of boulder-clay buried it. Now in Arran, as I have men- 

 tioned, I found several cases of just such a bed resting directly on the rock, and 

 buried beneath the boulder- clay ; and the inference seemed to me a fair one, that 

 these beds indicated a sudden subsidence of the land at that point. I still believe 

 I, them to indicate the remoteness of the ice-cake at the time of their deposition, 



* Such jerks must of course be supposed considerable, since, if minute, the subsidence in this 

 i way would practically be slow and steady as in the other. 



