THE DEEP SEA AND ITS CONTENTS. 343 



the bottom of the cHff. Sh'ght irregularities in the general 

 parallelism of the stratification, and the occasional thinning-out of 

 particular lamella;, were easily accounted for by the drifting of the 

 snow-layers of the surface, before they had become consolidated. 

 And although there are various cases in which the strata had 

 been changed from their original horizontality to various degrees 

 of inclination, sometimes also being traversed by " faults," and 

 occasionally even twisted and contorted, these might all be 

 accounted for by forces acting subsequently to the detachment 

 of the bergs. For their plane of flotation is liable to altera- 

 tion by changes of form due to unequal melting, and the sepa- 

 ration of large masses either above or below the surface ; and 

 " dislocations " of various kinds will be produced by collisions 

 and lateral thrusts, when bergs are impelled against each other 

 by the wind. Sir Wyville Thomas and Mr. Moseley entirely 

 agree in the statement that they could nowhere trace any such 

 " structure " as is produced in a land-glacier during its move- 

 ment down a valley, by the curvature and contraction of its 

 rocky borders, and the inequalities of the bottom over which it 

 moves. And the presumption is altogether very strong that these 

 vast masses have originally formed part of a great ice-sheet, formed 

 by the cumulative pressure of successive snow-falls over a land 

 area of no great elevation ; which flows downwards from its highest 

 level in the direction of least resistance, that is to say from the 

 Polar centre towards the continually disintegrating margin, pro- 

 gressively diminishing in thickness as it extends itself peripherally. 

 Thus gradually moving seawards, the ice-sheet will at last pass 

 the margin of the land, but will continue to rest upon the gradually 

 descending sea-bed, flowing down its gentle slope until lifted by 

 its own buoyancy (like a vessel or launch), when vast masses will 

 break off and float away. 



Although the observers of the Challenger did not see either 

 masses of rock, stones, or even gravel upon any of the icebergs 

 they approached, VVilkes and Ross saw many such : and the 

 "soundings" of the Challenger were found to consist of such 

 comminuted clays and sands as would be the result of the abrasion 

 of rocky surfaces over which the ice-sheet had moved ; while 

 the dredge brought up a considerable quantity of land debris — 



