5iS HISTORY OF GEOLOGY. 



repeatedly seen icebergs and icefloes sailing along laden with sucL 

 materials. 



The above explanation of the phenomena of drift supposes the land 

 on which the travelled materials are found to have been* the bottom 

 of a sea where they were deposited. But it does not, even granting 

 the conditions, account for some of the facts observed ; that the drift 

 and the boulders are deposited in "trainees" or streaks, which, in 

 direction, diverge from the parent rock ; and that the bonlders are 

 of smaller and smaller size,' as they are found more remote from that 

 centre. These phenomena rather suggest the notion of currents of 

 water as the cause of the distribution of the materials into their 

 present situations. And though the supposition that the whole area 

 occupied by drift and boulders was a sea-bottom when they were scat- 

 tered over it much reduces the amount of violence which it is neces- 

 sary to assume in order to distribute the loose masses, yet still the 

 work appears to be beyond the possible effect of ordinary marine cur- 

 rents, or any movements which would be occasioned by a slow and 

 gradual rising of the centre of distribution. 



It has been su<wested that a sudden rise of the centre of distribution 



oo 



would cause a motion in the surrounding ocean sufficient to produce 

 such an effect : and in confirmation of this reference has been made 

 to Mr. Scott Russell's investigations with respect to waves, already re- 

 ferred to. (Book vni.) The wave in this case would be the wave of 

 translation, in which the motion of the water is as great at the bottom 

 as at the top ; and it has hence been asserted that by paroxysmal ele- 

 vations of 100 or 200 feet, a current of 25 or 30 miles an hour might 

 be accounted for. But I think it has not been sufficiently noted that 

 at each point this " current" is transient : it lasts only while the wave 

 is passing over the point, and therefore it would only either carry 

 a single mass the whole way with its own velocity, or move through 

 a short distance a series of masses over which it successively passed. 

 It does not appear, therefore, that we have here a complete account 

 of the transport of a collection of materials, in which each part is 

 transferred through great distances : except, indeed, we were to sup- 

 pose a numerous succession of paroxysmal elevations. Such a battery 

 might, by successive shocks, transmitting their force through the 

 water, diffuse the fragments of the central mass over any area, however 

 wide. 



The fact that the erratic blocks are found to rest on the lower drift, 

 is well explained by supposing the latter to have been spread on the 



