368 Mr. J. Croll on Geological Time, and the probable 



20 feet (and this is perhaps more than any ocean-current could 

 effect), would only exert a pressure of about half a ton on the 

 square foot, or about ^ part the pressure of the glacier 1000 feet 

 in thickness. A great deal has been said about the erosive and 

 crushing-power of icebergs of enormous thickness, as if their 

 thickness gave them any additional pressure. An iceberg 100 

 feet in thickness will exert just as much pressure as one a mile 

 in thickness. The pressure of an iceberg is not like that of a 

 glacier, in proportion to its thickness, but to the height to which 

 it is raised out of the water. An iceberg 100 feet in thickness 

 raised 10 feet will exert exactly the same pressure as one a mile 

 in thickness raised to an equal height. 



To be an efficient grinding agent, steadiness of motion, as well 

 as pressure, is essential. A rolling or rocking motion is ill-adapted 

 for grinding down and striating a rock. A steady rubbing 

 motion under pressure is the thing required. But an iceberg is 

 not only deficient in pressure, but also deficient in steadiness of 

 motion. When an iceberg moving with considerable velocity 

 comes on an elevated portion of the sea-bottom, it does not move 

 steadily onwards over the rock, unless the pressure of the berg 

 on the rock be trifling. The resistance being entirely at the 

 bottom of the iceberg, its momentum, combined with the pres- 

 sure of the current, applied wholly above the point of resistance 

 tends to make the berg bend forward, and in some cases upset 

 (when it is of a cubical form). The momentum of the moving 

 berg, instead of being applied in forcing it over the rock against 

 which it comes in contact, is probably all consumed in work 

 against gravitation in raising the berg upon its front edge. 

 After the momentum is consumed, unless the berg be completely 

 upset, it will fall back under the force of gravitation to its ori- 

 ginal position. But the momentum which it acquires from gra- 

 vitation in falling backwards carries it beyond its position of re- 

 pose in an opposite direction. It will thus continue to rock 

 backwards and forwards until the friction of the water brings it 

 to rest. The momentum of the berg, instead of being applied 

 to the work of grinding and striating the sea-bottom, will chiefly 

 be consumed in heat in the agitation of the water. But if the 

 berg does advance, it will do so with a rocking unsteady motion, 

 which, as Mr. Couthouy* and Professor Dana f observe, will tend 

 rather to obliterate striations than produce them. 



A floating berg moves with great steadiness ; but a berg that 

 has run aground cannot advance with a steady motion. If the 

 rock over which the berg moves offers little resistance, it may 



* Report on Icebergs, read before the Association of American Geologists. 

 Silliman's Journal, vol. xliii. p. 163 (1842). 

 t Manual of Geology, p. 677* 



