72 THE WONDERS OF GEOLOGY. Lect. I. 



immense masses of rocks and boulders by drifting ice-floes, 

 and the distribution of these materials in the bed of the 

 ocean, or on the land, as the congealed -water gradually 

 melts as it approaches a milder climate — are phenomena so 

 familiar to the English reader, from the graphic accounts 

 which have emanated from our naval officers engaged in 

 the arduous service of the voyages of discovery, undertaken 

 during the present century, that it will not be necessary to 

 dwell upon the subject. 



Glaciers are sheets of ice, which form upon the tops and 

 in the clefts and valleys of mountains, that attain such an 

 elevation as to be within the regions of eternal snow. They 

 are not inaptly termed, by that most charming of all philo- 

 sophical travellers, Mr. Darwin, gigantic icicles. The 

 lowest limit to which glaciers extend, depends, of course, 

 on the height of the line of perpetual congelation, which 

 varies in different latitudes, and is also modified by the 

 configuration of the land. In the Alps the traveller enters 

 the region of eternal snow at a height of from 3000 to 7000 

 feet ; while in South America, under the equator, the snow 

 line is equal to that of the summit of Mont Blanc* In 

 the Alps there are vast table-lands, of from 100 to 300 

 square miles, composed of solid ice; from which descend 

 enormous masses, that slowly glide towards the lower 

 regions, bearing with them innumerable fragments of rock. 

 So soon as these glaciers reach the lowest limit of perpetual 

 congelation, they melt, and the fragments of rock they 

 contain are deposited, and form mounds and ridges of stones 

 of all sizes, which are called moraines; these remain after all 

 vestiges of the agent which produced them have disappeared. 

 The difference in the character of the materials brought 

 down by torrents and by glaciers is very manifest : for the 

 former give rise to beds of gravel ; the latter to ridges or 



* Humboldt. 



