Ixiv PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



Now, adopting all the mathematical and hydrostatical calculations 

 of Mr. Hopkins as correct, they prove, I think, the non-occurrence 

 or extreme rarity in past times of earthquake-shocks more violent 

 than such as we have experienced in the last ten centuries. For when 

 we consider how many marine formations have been upraised, some 

 of them from seas of considerable depth, and what a vast amount of 

 upheaval and subsidence, estimated, as I have already reminded you, 

 by miles vertically, has taken place, it seems clear that if currents and 

 waves of such power as those contemplated by Mr. Hopkins had really 

 been set in motion, there would have been erratic blocks in deposits 

 of all ages, instead of their being confined to the close of the tertiary 

 period. Had these mighty waves swept again and again over the floor 

 of the ocean, and over the land in ancient periods, a drift or boulder 

 clay with rounded and angular blocks would have been conspicuous in 

 the Eocene, Cretaceous, Jurassic, Triassic, Permian, Carboniferous, De- 

 vonian and Silurian formations, and would have been most strikingly 

 displayed in such of these epochs as have been of the longest duration. 

 I have seen fragments of gneiss eight feet in diameter in the base of 

 the Silurian series in Canada, in the group called by the New York 

 geologists the Potsdam sandstone* ; but I observed in the same place 

 similar gneiss in situ, in the immediate vicinity, so that the blocks 

 may have been detached from an undermined cliff of the Silurian 

 sea-coast. In like manner, in the valley of the Bormida, in Pied- 

 mont, there are huge rounded masses of serpentine in the tertiary 

 molasse ; but similar rocks in situ pre-existed in the same region, so 

 that blocks may have been derived from the destruction of cliffs close 

 at hand. In Scotland, also, we see occasional fragments of large 

 dimensions in the conglomerates of the old red sandstone, especially 

 on the western coast, but in that case there is no ground for presuming 

 distant transport. In no part of the geological series, except in that 

 of very modern date, do we find an extensive deposit of drift, Hke 

 that spread over Northern Europe and North America. 



It may doubtless be objected, that by adopting the glacial hypothesis 

 we concede the possibility of one natural agent, such as frost, acqui- 

 ring at certain periods an intensity of action far greater than at others, 

 and hence I may be asked, whether the energy of any other cause 

 may not in an equal degree be subject to secular variation ? I admit 

 * See Travels in North America, vol. ii. p. 126. 



