97 



[Lesley. 



The number of stones thus inoperative at the base of a glacier is one of 

 the factors in the equation of erosion. 



That ice uses sand, gravel and boulder debris to scratch its rock bed is 

 not doubted by any one. The abrupt termination of striae, deepening and 

 widening to their abrupt termination, was one of the earliest observed 

 facts, and was explained on the old diluvial theory, and the iceberg theory, 

 by the arrest and rotation of the block which served as a graving tool, 

 fixed in the ice, or by the breaking off of the point of the tool. 



The chapters of James Hall's Report of the Geology of the Western Dis- 

 trict of New York, published in 1844, which describe the Drift and Glacia- 

 tion of that District suffice to show how carefully these phenomena were 

 studied fifty years ago. Dr. Newberry and other ultra-erosionists would 

 do well to note what Hall says (on page 331) in evidence of the compara- 

 tively slight force necessary for producing the grooves and polished sur- 

 faces, the overturning of bed plates, and transport of fragments, from 

 which such exaggerated theoretical consequences are deduced. 



In those really admirable chapters may be found the earliest hints of the 

 now accepted activity of subglacial water, loaded with debris, in doing 

 much of the work wrongly ascribed to ice. 



The actual erosive power of I'ock-set ice must certainly be susceptible of 

 an approximately accurate mathematical calculation. 



Its differential is : one stone, held by the ice against the bed-rock with a 

 certain pressure — the stone of a certain hardness (a) — the bed-rock of a 

 certain hardness (b) — the ice-grasp of the stone, of a certain plasticity (c) 

 — the maximum pressure exerted by the weight of the ice, up to the point- 

 crushing degree (d). 



It is evidently wrong to make the total weight of the column of ice above 

 the tool a measure of the engraving. Were the ice piled to the height of 

 miles, its graving power would be no greater than that of a column of ice 

 weighing just enough to crush the point of the tool. All the declamation in 

 books respecting the enormous erosive force of a sheet of ice several thou- 

 sand feet thick pressing down upon and moving over sandstone, limestone 

 and shale strata is simply wasted. A thousand miles thickness of pure ice 

 moving over a bed of clay, would erode it no more than a thousand miles 

 of water would. If it held stones, they would be simply embedded in the 

 clay and left behind. If they moved over any kind of solid rock, they 

 would simply be reduced to fine sand or mud, and act as a lubricating 

 medium, protecting the bed-plate surface from erosion. 



Every glacier must slip to a greater or less extent upon a lubricated sur- 

 face, consisting principally of muddy water, or watery mud. The thicker 

 the glacier the more of this lubricant it will have beneath it. The 

 law of increase of temperature descending from the surface must 

 act in ice as in rock. Where the bare rock surface of the earth has 

 a mean temperature of 33°, the temperature at 1000 feet down stands 

 at, say, 530. Were a glacier 3000 feet deep to remain for a century 

 immovable over a region the normal mean air temperature of which is 



