496 REVIEWS THE STORY OF A BOULDER. 



Wherever a mountain is sufficiently lofty to pierce this upper stratum, its summit 

 is covered with snow, and, as the snow never melts, it is plain that, from the accu- 

 mulations of fre^h snow drifts, the mountain-tops, by gradually increasing in height 

 and width, would become the sii^porting columns of vast hills of ice, which break- 

 ing up at last from their weight and width, would roll down the mountain-sides 

 and cover vast areas of country with a ruin and desolation more terrible than that 

 of any avalanche. Olympus would really be superposed upon Ossa. By a beau- 

 tiful arrangement this undue growth is prevented, so that the hill-tops never vary 

 much in height above the sea. The cone of ice and snow ■which covers the higher 

 part of the mountain, sends down into each of the diverging valleys a long slug- 

 gish stream of ice, with a motion so slow as to be almost imperceptible. These 

 streams are called glaciers. As they creep down the ravines and gorges, blocks of 

 rock detached by the frosts fi'om the cliffs above, fall on the surface of the ice, 

 and are slowly carried along with it. The bottom also of the glaciers is charged 

 with sand, gravel, and mud, produced by the slow-crushing movement; large 

 rocky masses become eventually worn down into fragments, and the whole surface 

 of the hard rock below is traversed by long parallel grooves and striae in the 

 direction of the glacier's course. Among the Alps, the lowest point to which the 

 glacier descends is about 8500 feet. There the temperature gets too high to allow 

 of its further progress, and so it slowly melts away, choking up the valleys with 

 piles of rocky fragments called moraines, and giving rise to numerous muddy 

 streams that traverse the valleys, uniting at length into great rivers such as the 

 Rhone, which enters the Lake of Geneva, turbid and discoloured with glacial 

 mud. 



In higher Lititudes, whe; e the lower limit of the snow-line descends to the level 

 of the sea, the glaciers are often seen protruding from the shore, still laden with 

 blocks that have been carried down from valleys far in the interior. The action of 

 storms and tides is sufficient to detach large masses of the ice, which then floats 

 off, and is often wafted for hundreds of miles into temperate regions, wher« it 

 gradually melts away. Such floating islands are known as icebergs. '. 



II. In climates such as that of Canada, where the winters are very severe, the 

 rivers become solidly frozen over, and, if the frost be intense enough, a cake of 

 ice forms at the bottom. In this way sand, mud, and rocky fragments strewing the 

 banks or the channel of the stream, are firmly enclosed. When spring sets in, and 

 the upper parts of ti.e i-ivers begin to thaw, the swollen waters burst their wintry 

 integuments, and the ice is then said to pack. Lajer is pushed over layer, and 

 mass heaped upon mass, until great floes are formed. These have often the most 

 fantastic shapes, and are borne down by the current, dropping, as they go, the mud j 

 and boulders, with which they are charged, until they are stranded along some 

 coast Ime, or melt away in mid-ocean. 



III. But icebergs are also produced by the freezing of the water of the ocean. 

 In high latitudes, this takes place when the temperature falls to 28.5° of Fahren- 1 

 heit. The surface of the sea then parts with its saline ingredients, and takes the j 

 form of a sheet cf ice, which, by the addition of successive layers, augmented 

 sometimes by snow-drifts, often reaches a height of from thirty to forty feet. 

 On the approach of summer these ice-fields break up, crashing into fragments I 



