1845.] SMITH ON SCOTCH BOULDERS. 37 



this deposit was permanently submerged about the period to which 

 we must refer these phsenomena, viz. about the end of the tertiary 

 period. These icebergs however must have obeyed every impulse 

 of wind and tide ; and when we find angular masses transported from 

 a distance resting on the surface or imbedded in marine strata, we 

 may infer with great probability that they were dropped from ice- 

 bergs : this also is an exceptional case, of which I have never met 

 an instance. But even if such phsenomena were much more frequent 

 than we find them to be, they would throw no light whatever on the 

 origin of the boulder clay. The explanation which is attended with 

 fewest difficulties is that propounded by Sir James Hall, and by one 

 and all of the early Scotch observers, and maintained with great 

 ability and knowledge of detail by Mr. Milne, in his valuable papers 

 already referred to, the first of which indeed has no reference to the 

 glacial theory, which was not then propounded, at least in this 

 country. This explanation, modified by later discoveries, is as fol- 

 lows : — A rush of water, such as that produced by earthquake waves 

 of sufficient violence to tear up not only the pre-existing unconsoli- 

 dated cover, but considerable portions of the subjacent rocks, and 

 perhaps obliterate the inequalities caused by disturbances in the coal- 

 measures, passed over the island from west to east, or rather from the 

 north-west, depositing the whole in a confused mass on the surface. 

 In that part which was under the sea, beds of gravel, sand and clay 

 were deposited. In process of time a second debacle again swept 

 over the island in the same direction, but with much less violence 

 than the first ; the stratified beds, perhaps of no great thickness, were 

 swept away, leaving however occasional patches, sufficient to attest 

 their existence, and also part of the pre-existing diluvium, reducing 

 the inequalities and grinding the exposed surfaces of the rocks and 

 boulders ; for it is to this second debacle I ascribe the scratching of 

 the rocks and boulders ; and here 1 think ice acted an important 

 part, and was probably the principal agent in grinding down the 

 substances over which it passed : a colder climate and a north-west 

 direction both point to a frozen ocean which was perhaps broken up 

 by the convulsion which caused the diluvial wave, and the ice of 

 which was swept over the land in the same direction. That there 

 are two separate deposits of till, I have no doubt ; the newer is finer, 

 lighter in the colour, and with fewer and smaller boulders than the 

 older, and when seen in contact the junction is well-defined; such a 

 junction may be seen in the. cover of Gilmour Hill quarry, near 

 Glasgow. Mr. Craig informs me that he found the femur of the fossil 

 elephant in stratified beds between these separate deposits ; and Mr. 

 Milne describes them under the names of the " upper covering of 

 gravel and boulders," and the "lowest boulder clay," enumerating 

 four stratified beds between them. M. d'Archiac, in his account of 

 the Geology of the Department de I'Aisne, also notices two unstra- 

 tified deposits, both of which he ascribes to diluvial action. 



