CAUSES OF DRIFT. 73 



instance — most of the drift has been thus produced, and most of the erratic blocks 

 thus scattered. 



This agency, too, we can trace back to the dawn of the drift period, and it is 

 still in operation on a stupendous scale in arctic and antarctic regions. What we 

 witness of its effects in temperate seas shows only its power in transporting afar 

 blocks of stone which it has torn from the shore. 



3. Mountain Slides produced by Aqueous Agency. — If any one doubts whether 

 this should be reckoned among the causes of drift, let him visit the case described 

 in this paper, on what I call Moraine Brooh, in Mount La Fayette, at Franconia, and 

 he will see first, that the materials torn off from the ledges and strewed along for 

 two miles, cannot be distinguished from coarse drift ; and secondly, that they are 

 so arranged as not to be distinguishable from the lateral and terminal moraines of 

 a glacier. Why then should they not be regarded as drift ? 



4. Waves of Translation, produced by the Paroxysmal Upheaval of Continental 

 Masses, or Earthquake Undulations. — Whether any certain example of such a move- 

 ment can be pointed out — unless we admit drift generally to have been thus pro- 

 duced — I exceedingly doubt. But hypothetically we can realize that such waves 

 would tear off fragments of rocks and roll them along and smooth, if they did not 

 groove, the rocks. This action would, indeed, be too short, violent, and irregular, 

 to explain all the features of drift, which were the work of agents acting ages 

 upon ages : yet, from the phenomena occasionally exhibited by earthquake waves 

 along the coast, we may reasonably include this force among those concerned in 

 the production of drift. And in some countries it may have done much of the 

 drift work. 



5. Perhaps I ought to add, as a fifth cause of drift, those ice floods that occur 

 almost every winter in the rivers of northern and mountainous countries. Often 

 in these cases, the river is choked with fragments of ice, so that its banks are full. 

 Yet there is water enough to keep it slowly in motion. It differs, in fact, from a 

 glacier, only in being more fluid, so that its motion is more rapid. But it grates 

 powerfully upon the sides and bottom of the stream, and produces miniature 

 moraines. I see not why such detritus should not be regarded as drift as much as 

 the moraines of glaciers or icebergs. 



42. According to these views, drift is the result of several agencies that have 

 been in operation upon the earth's surface, certainly since the tertiary period, and 

 in some countries, from a much earlier date. They have varied in intensity at 

 different times, and in different circumstances, and each one has had a predomin- 

 ance at certain times. But all of them are still in action in some parts of the 

 globe, and perhaps with as much power as ever. 



43. In like manner, alluvial agencies have had an operation parallel to those 

 producing drift, and as far back, though the present forms of alluvium are chiefly 

 posterior to the tertiary epoch. But perhaps the whole formation is not so. 



44. Drift and alluvium ought to be regarded as only varieties of the same forma- 

 tion. And since water has always been present and essential in the operation of 

 the other agencies, the whole formation should take the name of alluvium. Chro- 



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