542 REV. R. B. WATSON ON THE GREAT DRIFT-BEDS WITH SHELLS 



but I no longer think that remoteness of the ice due to sudden subsidence. Had 

 it been so, the basement-bed of sand and clay would have run round our whole 

 coast pretty much at the same level. But this is not true, even for Arran. The 

 stratified beds, therefore, on the basement rock must, where they exist, be 

 accepted merely as local phenomena, and be therefore explained by local causes, 

 connected probably with those banks of eruptive rock which lie across the valleys, 

 and Avhich must have interfered with the free motion of the ice. 



Thus, in the absence of any evidence of a sudden depression we accept the 

 other alternative of steady gradual subsidence, during which each separate level 

 must in turn have formed the shore line and lain at the edge of the ice foot, and 

 have received the glacial detritus — the coarse red clay and stones which the ice 

 and its accompanying streams were bringing down — while the finer detritus 

 would be spread as stratified sands and gravels over the boulder-clay beds 

 already deposited. 



10. Was the subsidence rapid ? A question I rather ask than answer. If 

 the mass of the boulder-clay suggest long-continued formation, the comparative 

 rarity of interstratified beds, and the merely local development of the overlying 

 beds of sand and clay — though perhaps capable of explanation on other grounds 

 — still seem to point to such a steadiness of climate and of currents as is hardly 

 compatible with a very lengthened duration of the glacial epoch ; and in this 

 case the huge mass of the glacial debris would be the result of an enormous 

 development of the ice-cake, which tallies well enough with various known facts. 

 On the whole, this point must be considered doubtful, and its determination is 

 probably to be sought from a careful examination of the larger shell-beds, the 

 layers of life in which may indicate the duration of their development. 



11. There has been no general glaciation of the land since its re-emergence. 

 Had glaciers existed on the land after it rose from the sea, they would certainly 

 have cleared the upper valleys at least, of boulder-clay, and left nothing but great 

 transverse moraines, as is the case in Norway. (See Kjerulf on the glacial phe- 

 nomena of Norway in the Edinburgh New Phil. Journal for July 1863, p. 8.) 

 Instead of this the boulder-clay thins out gradually upwards, and fringes the 

 upper valleys as a beach terrace. 



12. The existing boulder-clay must represent all ages throughout the whole 

 glacial epoch ; for so long as the ice-cake was present on any part of the land the 

 manufacture of boulder-clay went on. 



13. There must be drift-beds of sand and clay resting on the boulder-clay, 

 but truly contemporary with the boulder-clay of a higher level than that on 

 which they lie. It may not be easy or possible to determine which of these 

 answer to one another ; but it is obvious, that while the coarser debris from the 

 land was forming boulder-clay under the ice- foot, a deposition of the lighter 

 detritus must have been going on from the fresh-water currents that were setting 



