The Glacial Theory of Prof . Agassiz. 357 



down as Martigny. The lake of Distel, on the Saas, those of Rufnen 

 and Gurglen, in the Tyrol, and that of Passey, on the Adige, are also 

 formed by glaciers. The last has burst its dike six times, with the most 

 destructive effects, since 1404. Agassiz traced decided marks of an 

 ancient glacier at the north side of Ben Nevis. This glacier, he thinks, 

 had closed up the valley of the Spean, and formed a lake in Glenroy, 

 in which the banks of gravel, called Parallel Roads, were deposited. 

 The barrier being of ice, which subsequently melted, the absence of 

 any marks of its existence is accounted for. At present we shall not 

 stop to inquire whether this theory or Mr. Darwin's is the more probable. 



Alluvial Deposits. — Agassiz thinks that the floods produced by the 

 bursting of such lakes as those described, and by the fusion of the ice, 

 tore up the moraines, scattered their materials over the country, and 

 formed the unstratified boulder clay, and the stratified sand and gi-avel 

 resting upon it, which now cover nearly the whole surface of the low 

 country. 



Ancient extent of Glaciers in Sioitzerland. — The traces of ancient 

 lateral moraines are seldom very distinct ; yet in the lower valleys, 

 where no glaciers now exist, in that of the Rhone, for instance, between 

 Martigny and the lake of Geneva, several may be seen ranged in par- 

 allel lines, one above another, at 1000, 1200, and even 1500 feet above 

 the river. Terminal moraines are found half a mile, a mile, a league, 

 and even several leagues from existing glaciers ; but these are in se- 

 condary valleys, and belong to the period when the glaciers were re- 

 treating into the narrow limits which they now occupy, while the floods 

 which occurred at this period had obliterated those of the principal 

 valleys. The striated and polished surfaces, which had a more dura- 

 ble existence, are found at great heights ; among other examples, on 

 Seidelhorn, (an isolated mountain in the Alps, now destitute of glaciers,) 

 2590 feet above the bottom of the valley, indicating that ancient gla- 

 ciers of this depth or more existed here. The boulders also, or blocs 

 perches, the creux or pits, and the lapiaz or water-worn gutters, were 

 all observed far beyond the present limits of the glaciers. This first 

 step in the argument conducts Agassiz to the conclusion that the whole 

 of the Alps, at sojne ancient period, formed one vast mer de glace, the 

 ice descending to the level of the great Swiss valley which separates 

 these mountains from Jura. 



But the same indications of glacial action exist on Mount Jura, which 

 runs parallel to the Alps, divided from them by the great Swiss valley, 

 fifty miles in breadth. This chain, which is of moderate height, is 

 now entirely destitute of glaciers, and, owing to the nature of the rock, 

 the marks of abrasion are remarkably numerous and distinct. They are 

 found on the side fronting the Alps from the bottom to the summit, and 



