TWO GLENS AND THE AGENCY OF GLACIATION. 195 



Cap. Such a body could not have failed, by its vast pressure, to have ground down all 

 the surfaces on which it rested, or over which it passed ; and the appearances actually 

 presented are generally quite distinctive of an agency much lighter and more passing 

 in its work. The same lesson is taught by another phenomenon very common on 

 our hills ; and that is the position of enormous boulders and masses of rock left poised 

 or "perched" upon the very tops of ridges, isolated hills, in such a manner that no 

 conceivable agency but that of floating ice-floes could have placed them and left them 

 where we now see them. Nor are these blocks deposited only on the tops of conspicuous 

 hills, but also on knolls and elevations of every sort and kind, just as would naturally 

 happen on shoals and banks in a rising or falling sea. Moreover, it is to be observed that 

 many of these stones are not rounded, as they must have been if they had been rolled 

 under water, or dragged along in the lower layers of a moving glacier. Many of them 

 are rough, and even angular in a high degree — just as they might have fallen from some 

 overhanging cliff, or have been torn off by the splitting power of frost. These are all 

 arguments to show that even if an ice sheet had ever existed as a moving mass, it could 

 not have produced the phenomena which we actually see. But besides these arguments, 

 there are others which condemn the supposed ice sheet as a physical impossibility — 

 inasmuch as no adequate cause of motion has ever been made out for such an assumed 

 " flow " of such ice-masses. 



Putting all these considerations together, I had long; come to the conclusion that our 

 glaciation has been effected mainly by ice-floes and occasional icebergs in a glacial sea, 

 which rose at least some 2000 feet above the level of our present ocean, and in which 

 powerful currents were running in a general direction from N.E. to S.W. There is a 

 natural and legitimate aversion to such an explanation. It defies altogether that 

 impression of the stability of the relative position and levels of sea and land, which is one 

 of the strongest preconceptions we derive from our own uniform experience. Forgetting 

 how very short that experience is, and how inadequate to justify any conclusions as to an 

 unknown past, our preconception is farther helped by the extreme difficulty of even 

 imagining any physical cause for a submergence of the land, which would seem to have 

 been so recent and so passing. These are excellent reasons for reserve and caution in our 

 reasoning, but they are no excuses for reluctance in admitting the evidence of obvious 

 facts, or for carelessness in making closer and closer observation as to facts which may 

 not be equally apparent at first sight. After all, we must remember that geology has 

 made us familiar with the idea of the interchangeability of sea and land, almost 

 everywhere over the globe, as one of its most certain facts ; and the assumptions, so often 

 tacitly made, that all those changes must have been always infinitesimally slow, are 

 assumptions in the highest degree precarious, and founded on theories for which there is 

 really no adequate foundation. We must remember also that the movements which arc- 

 suggested, although they startle us by their evident recency in point of time, and by our 

 conception of their magnitude in elevation and depression, are, after all, movements of 

 infinitesimal smallness when considered with reference to the size of the globe, and with 



