IN THE SOUTH OF ARRAN. 543 



seaward, and scattering, first the sand and ultimately the mud, with which they 

 were charged, on the boulder-clay already spread over the lower levels. Thus 

 these two beds, the boulder-clay formed under the ice-foot, and the stratified 

 sands and clays deposited in deeper water, though so different in texture, would 

 be really strictly contemporary. Of course the determination in any particular 

 case, of which are the corresponding beds, is now the more difficult task, from 

 the erosion of the beds at certain points, and their remaniement or remanufacture 

 in others. 



14. We may determine, approximately at least, the relative age of the drift- 

 beds. Superposition, of course, implies subsequence in time, but this principle 

 is of very limited application, and will not avail for the comparison of the 

 boulder-clay in different districts. If, however, the boulder-clay was deposited 

 under the ice-foot as the land was subsiding, the sea-level affords us a standard 

 of comparison for the boulder-clay beds over the whole country. Allowing for 

 possible differences in the thickness of the ice-foot, which would be deeper of 

 course in the valleys than on the hill faces, and deeper also in a mountainous 

 than in a flat region, all boulder-clay beds are contemporary which rest on the 

 basement rock at the same level above the .sea ; and of two beds at different 

 levels that is the older which lies on the rock nearest the sea-level. 



15. Since the boulder-clay period there has been no material change of any 

 kind on the basement rock of the country. 



Such changes might have occurred in three ways. In the process of subsidence 

 and re-elevation the whole face of the land might have been remodelled, and 

 hill and valley have changed places. Such a change, or something like it, has been 

 asserted even by so eminent an authority as the great German geologist Naumann. 

 (See Geognosie, vol. i. p. 249.) But in our drift-beds we have casts made of our 

 valleys as they sank under the sea, and these casts show that what are valleys 

 now were valleys then ; in other words, they assure us that the subsidence and 

 re-elevation of the land has not been accompanied by any such protrusion of one 

 part of the coast above another, or of the interior above the coast line, as to 

 affect the relative contours of hill and valley. 



Another form of change is that on the river beds, the erosion of which is 

 generally attributed to existing streams ; whereas in Arran we find that the 

 burns are only now beginning to lay bare the rock which underlies the boulder- 

 clay. If, then, making all allowance for the slight erosive power of such small 

 streams, they have yet done so little here against sands and clays, how much 

 less elsewhere against solid rock. They seem, in fact, to be only now beginning 

 to occupy the old river-beds formed long ago under the ice, and in part even 

 earlier. 



The third change which has been often asserted, consists in the erosion of 

 the older and present coast-lines by the sea, upon which calculations have even 



VOL. XXIII, PART III. 7 H 



