TRANSACTIONS OF SECTION E. 621 



like over the land, just reaches the layer at which the temperature of the water 

 distinctly rises ; and is rapidly melted, and the pehhles and land debris with which 

 it is more or less charged are precipitated. That this precipitation takes place all 

 over the area where the icebergs are breaking up, constantly and to a considerable 

 extent, is evident from the fact that the matter brought up by the sounding instru- 

 ment and the dredge is almost entirely composed of such deposits from ice; for 

 Diatoms, Foraminifera, and Radiolarians are present on the surface in large num- 

 bers, and unless the deposit from the ice were abundant it would soon be covered 

 and masked by the skeletons of surface organisms. 



The curious question now arises, what is the cause of the uniform height of the 

 southern icebergs, — that is to say, what is the cause of the restriction of the 

 thickness of the free edge of the ice-cap to 1400 feet? I have mentioned the 

 gradual diminution in thickness of the strata of ice in a berg from above down- 

 wards. The regularity of this diminution leaves it almost without a doubt that 

 the layers observed are in the same category, and that therefore the diminution is 

 due to subsequent pressure or other action upon a series of beds, which were at the 

 time of their deposition nearly equally thick. About sixty or eighty feet from the 

 top of an iceberg, the strata of ice a foot or so in thickness, although of a white 

 colour, and thus indicating that they contain a considerable quantity of air, are 

 very hard, and the specific weight of the ice is not much lower than that of layers 

 three inches thick nearer the water-line of the berg. The upper layers have been 

 manifestly produced by falls of snow after the berg has been detached. 



Now, "it seems to me that the reduction in thickness cannot be due to com- 

 pression alone, but that a portion of the substance of the lower layers must have 

 been removed. It is not easy to see why the temperature of the earth's crust, 

 under a widely extended and practically permanent ice-sheet of great thickness, 

 should ever fall below the freezing-point ; and it is a matter of observation that at 

 all seasons of the year vast rivers of muddy water flow into the frozen sea from 

 beneath the great glaciers which are the issues of the ice-sheet of Greenland. Ice 

 is a very bad conductor, so that the cold of winter cannot penetrate to any great 

 depth into the mass. The normal temperature of the surface of the earth's crust, at 

 any point where it is uninfluenced by cyclical changes, is at all events above the 

 freezing-point, so that the temperature of the floor of the ice-sheet would certainly 

 have no tendency to fall below that of the stream passing over it. The pressure 

 upon the deeper beds of the ice must be enormous at the bottom of an ice-sheet 

 1400 feet in thickness — not much less than a quarter of a ton on the square inch. 

 It seems, therefore, probable that under the pressure to which the body of ice is 

 subjected, a constant system of melting and regelation is taking place, the water 

 passing down by gravitation from layer to layer until it reaches the floor of the ice- 

 sheet, and finally working out channels for itself between the ice and the land, 

 whether the latter be subaerial or submerged. 



I should think it probable that this process, or some modification of it, may be 

 the provision by which the indefinite accumulation of ice over the antarctic con- 

 tinent is prevented, and a certain uniformity in the thickness of the ice-sheet 

 maintained — that in fact ice at the temperature at which it is in contact with the 

 surface of the earth's crust within the antarctic regions cannot support a column of 

 itself more than 1400 feet high without melting. It is suggested to me by Pro- 

 fessor Tait that the thickness of the ice-sheet very probably depends upon its area, 

 as the amount of melting through squeezing and the earth's internal heat will 

 depend upon the facility of the escape of the water. The problem is, however, an 

 exceedingly complex one, and we have perhaps scarcely sufficient data for working 

 it out. 



The Fauna of the Deep Sea. — I can scarcely regret that it is utterly impossible 

 for me on this occasion to enter into any details with regard to the relations of the 

 abyssal fauna, the department of the subject which has naturally had for me the 

 greatest interest. Recent investigations have shown that there is no depth limit to 

 the distribution of any group of gill-bearing marine animals. Fishes, which from 

 their structure and from what we know of the habits of their congeners must 

 certainly live on the bottom, have come up from all depths, and at all depths the 



