DYNAMICAL GEOLOGY. 22$ 



Agassiz can see no sufficient evidence of any periodic 

 regularity in the advance and retreat of glaciers ; the variations 

 of the glaciers represent, in his opinion, the result of two 

 opposing forces the forward movement of the ice-masses 

 and the solvent action of the atmosphere. The precise dimen- 

 sions of a glacier are, he writes, essentially correlated with 

 climatic conditions ; a change of climate produces a corre- 

 sponding increase or diminution in the size of a glacier. 

 Agassiz regards the testimony in Switzerland as absolutely 

 convincing, that the Swiss Alps were formerly almost wholly 

 under ice. He contributes a wealth of observations on old 

 moraines, rows of blocks left in Alpine valleys, rock-scratches, 

 scarred limestone wastes, pot-holes (Gletschermiihle\ and the 

 erratics (Findltnge) irregularly scattered on the plain. A very 

 valuable account was given by Agassiz of the original home, 

 the course of travel, and the ultimate position assumed by 

 many of the famous " erratic " blocks in Switzerland. 



Not the least interesting portion of the work is that in which 

 Agassiz disposes of various erroneous explanations previously 

 given for "erratics " by geologists of authority the suggestion 

 of De Saussure and Von Buch that the erratics had been 

 transported by river-floods, the explosion theory of Silberschlag 

 and De Luc, the gliding theory of Dolomieu, and the drift 

 theory of Lyell. 



After brief reference to the observations of rock-scratches 

 and erratics made by Sir James Hall in Scotland, by Brong- 

 niart and by Nils Sefstrom in Scandinavia, Agassiz pro- 

 ceeds to enunciate his theory of the Ice Age. In conformity 

 with Cuvier's Catastrophal Theory, he supposes that at the 

 close of the accumulation of the geological formations 

 there took place repeated falls of temperature, and that 

 immediately before the Alpine upheaval the earth became 

 covered with a thick crust of ice. An enormous ice-sheet 

 extended over the greater part of Europe and across the 

 Mediterranean as far south as the Atlas mountains, over 

 Northern Asia and Northern America; above the ice-sheet 

 only the highest summits emerged. 



While the Alps were being upheaved, the icy crust still 

 mantled the rocks, and any fragments dismembered from the 

 solid rock during the movements fell upon the ice and were 

 carried away upon its surface. After the completion of Alpine 

 uprise the climate became milder, and as the ice melted, great 



