188 PHYSICAL GEOLOGY. 



sea, the extraordinary rise and fall and the great erosion will cease. 

 Tides tend to produce deep inlets, and ultimately channels and islands ; 

 thus their effects may closely simulate the forms of shore given by 

 glacial action. P. D. 



Shaler, Prof. N. S. Propositions concerning the Motion of Con- 

 tinental Glaciers. Proc. Boston Nat. Hist. JSoc. vol. xviii. pp. 126- 

 133 (pp. 129-133 were published in 1876). 



Convinced that N. America, N. of the parallel of 40°, was covered 

 by a deep mass of ice during the last glacial period, the author does 

 not believe that there was a continuous movement of its substance 

 outwards, and endeavours otherwise to account for the transport of 

 material. Starting from Mr. J. Thompson's conclusion that for each 

 atmosphere of pressure the freezing point of water is lowered 0°*0075 

 Cent., it is argued that the bottom-ice would melt where the ice-sheet 

 was thicker; the water thus produced would not escape upwards, 

 because as it rose the relief of pressure would cause it to freeze again, 

 but would be forced out to where the ice was thinner. There would 

 be a continual alteration of volumes and pressures in the parts of the 

 mass, tending to a frequent melting and freezing. By the escape, 

 sometimes sudden, of the water thus accumulated transportation of 

 material was effected, much of that which makes the American 

 moraines being water-worn. Glacial basins would be thus formed : — 

 In any hollow there would be much water and falling-in of ice, with 

 movement of water with sand, pebbles, &c., and consequent erosion, 

 the intensity of this action increasing as the hollow deepened, and the 

 only limit being when a pit in the ice-surface immediately above the 

 basin began to form. The basins would be elongated in the direction 

 of the line from the supply-point to the border of the sheet, since that 

 would be the direction of the outflow of the water. The theory is thus 

 applied to the case of the great lakes. The striae are accounted 

 for by supposing a true forward motion of the ice near the edge of the 

 ice-sheet, the whole region being in turn scratched on the retreat of the 

 ice. P. D. 



Topley,W. Sand-Dunes and Blowing Sand. Pop. Sci. Rev. vol.xiv. 

 pp. 133-142. 



Describes the mode of formation of sand dunes, and notes the chief 

 accumulations of blown sand around the English Coast. The sands 

 of the Atlantic Coast of Western Europe are described, especially the 

 Landes ; then some of the sands of the Mediterranean, and those of 

 the Sahara. The stores of water beneath the Sahara are mentioned, 

 and the erosive action of blowing sand is described. W. T. 



Tylor, A. On the Action and Eormation of Eivers, Lakes, and 

 Streams, with remarks on Denudation and the Causes of the great 

 Changes of Climate which occurred just prior to the Historical 

 Period. Geol. Mag. dec. ii. vol. ii. pp. 433-476, 35 woodcuts. 



The friction of ice upon ice during the motion of a glacier produces 

 water ; this water in freezing helps to excavate rock-basins ; but the 



