184 Notice of Prof. Forbes's Travels in the Alps. 



and was much impressed by the magnitude and variety of the ruins 

 which they transport, even where there is little declivity, and over 

 ground quite uneven. In a journey in 1816, he observed the masses 

 on the Jura, and he distinctly attributed them to glaciers which once 

 crossed the lake of Geneva and the plain of Switzerland, and he denies 

 that a current of water, however powerful, could have carried huge 

 llocs up steep declivities. That there has been great variation in 

 the extent of the glaciers is proved by the fact, that between the elev- 

 enth and the fifteenth centuries, passes in the mountains which are now 

 rarely traversed at all, were often threaded by the inhabitants on foot 

 and on horseback. 



It is well known that Venetz, De Charpentier, and especially Agassiz, 

 have given great extent to the glacial theory, and Agassiz " has at- 

 tempted to extend it with some variations to every part of the tem- 

 perate zone, and to explain the distribution of the Scandinavian blocks 

 and those of Great Britain by a similar action." 



Whatever may be thought of this extension, Prof. Forbes considers 

 the admission of glaciers one hundred miles long or more, as a less ex- 

 travagant hypothesis than might be imagined. The absence of such 

 a degree of cold in the existing climate, and the want of a sufficient 

 declivity, are obvious objections to this view. But " the quantity is 

 often so great as almost entirely to conceal the mass of the ice under 

 the prodigious load, which, during a long descent, is accumulated upon 

 them. 1 ' In some cases they fill up entire valleys. Masses may now 

 be seen on the glaciers, nearly or quite as large as those in the valleys. 

 The author estimated the magnitude of a block which he saw in the 

 valleys, as being nearly one hundred feet long and forty or fifty feet 

 high, and a fragment of slate in the valley of Saas pushed along by 

 the glacier of Swartzburgh, is estimated to contain two hundred and 

 forty four thousand cubic feet, which would require an average diam- 

 eter of nearly sixty two feet. Glaciers chafe and polish the rocks over 

 which they are pushed or dragged ; they wear down the solid granite, 

 or slate, or limestone. " They rub and wear and polish the rocks with 

 which they are in contact. Struggling to dilate, they follow all the sin- 

 uosities and mould themselves into all the hollows and excavations they 

 can reach, polishing even overhanging surfaces." 



Glaciers carry along with them masses of pulverized gravel and 

 slime, which, pressed by the enormous superincumbent weight of ice, 

 do effectually grind and smooth the surface of the rocky bed ; the tur- 

 bid water of the glaciers, charged with the fine material that has been 

 ground between the rock and the ice, exhibits the same appearance 

 from age to age. 



