PERIOD OF THE DRIFT. 69 



work of erosion went on for some time after the continent began to emerge. A 

 careful examination of the rounded and striated rocks at different altitudes, will 

 satisfy any one that in the valleys the work is considerably more fresh and less 

 affected by decomposing agencies than on high mountains. The erosions are also 

 deeper in the valleys. Sometimes, as on Holyoke, in Massachusetts, a succession 

 of vallej's crossing a mountain ridge, have been excavated, to a considerable 

 depth ; but I never saw any such drift valleys on the tops of high mountains. 

 All this looks as if the work at high altitudes was completed first, and continued 

 in the valleys after the emergence of the mountains. Yet, in this country, such 

 anachronism could not have been long continued, for in that case, the emergence 

 of the high mountains would have changed the direction of the abrading force 

 into the valleys, from a north and south direction, and this appears to have been 

 the case only to a limited extent. While only the higher parts of the mountains 

 were above the waters, as islands, they would not very much affect the direction 

 of the force, if it consisted of large icebergs. 



Some may imagine that rocks much elevated are more liable to surface disinte- 

 gration than when in valleys. This may sometimes be true, but I doubt whether, 

 with most rocks the reverse is not the fact. The best example of freshness in 

 rocks rounded and striated at high levels, that I have met with, may be seen on 

 the top of Monaclnoc, in New Hampshire, 3000 feet above tide water. Yet appa- 

 rently it is not as recent there as in the bottoms of some of the valleys. 



Upon the whole, I think that we must throw back the drift period with the 

 exception above named, at least as far as the time when this and other countries 

 were sinking beneath the ocean. But did the work take place during that sub- 

 sidence, or previous to it ? My own conviction is, that we have evidence that 

 the work extended into both those periods. If before the time of subsidence, it 

 was accomplished by glaciers on a former continent. If we find evidence, as I 

 think we do, in Wales, in Scotland, in some parts of Switzerland, and in New 

 England, that glaciers existed before the last submergence, the detritus accumu- 

 lated by them, although modified somewhat by oceanic action, ought to be l'egarded 

 as a part of the drift deposit. We know, also, that since the emergence of the 

 land, glaciers, in some countries, have been producing genuine drift. It is well 

 known that eminent men have referred the whole of the drift to glaciers, and 

 they seem to me to have proved uncontrovertibly, that the smoothing, rounding, 

 and striating of the rocks in northern regions, have been the result of large heavy 

 bodies of ice, forced along the surface by a vis a tergo. Now did the glacier 

 theory apply to other countries as well as to Switzerland, so far as my slight 

 examination of that country enables me to judge, I could not well resist its 

 adoption. But in Great Britain, and especially in this country, there are peculi- 

 arities in the drift phenomena, that lead me to hesitate, and inquire whether they 

 are not better explained by the passage over the surface of large icebergs and ice- 

 floes, whose effects scarcely differ from those of glaciers. Some of the reasons for 

 such an opinion are the following : — 



1. The occurrence of strise upon the northern slopes of mountains, even to a 



