Ch. XI.] SMOOTHED AND STRIATED ROCKS. 139 



of ice produced by the union of the two glaciers, forming what is called 

 a medial moraine. The number and position of these moraines will de- 

 pend on the number and size of the tributary glaciers which join the 

 main one. By such machinery, not only small stones and earth, but 

 erratic blocks of the largest size, are carried down from the mountains 

 to the lower valleys and plains, performing a journey of twenty or 

 thirty miles in the course of several centuries, and usually retaining 

 their edges sharp and Unworn to the last. 



When the glacier passes over uneven ground, it becomes rent, and 

 traversed by broad and deep transverse fissures, into which portions 

 of the lateral or medial moraines are precipitated. Kills of water also, 

 derived from the liquefaction of the ice by the sun's rays in summer, 

 run over the surface of the glacier until, arriving at one of these 

 fissures, they cascade into it. From this source, as well as from springs, 

 which must occasionally break out under the glacier, are derived 

 torrents which flow under the ice in tunnels, where the angular stones 

 which have fallen to the bottom through the fissures often become 

 rounded, as in the ordinary bed of a river. Other blocks and pebbles, 

 being fixed in the ice, and firmly frozen into it, are pushed along the 

 bottom of the glacier, abrading, polishing, and grooving the rocky floor 

 below, while each stone is reciprocally flattened, polished, and striated 

 on its lower side. As the forces of downward pressure and onward 

 propulsion are enormous, each small grain of sand, if it consists of 

 quartz or some hard mineral, scratches and polishes the surface, 

 whether of the underlying rock or of the boulder which impinges on 

 it, as a diamond cuts glass or as emery powder polishes steel. The 

 strise which are made, and the deep grooves which are scooped out by 

 this action, are rectilinear and parallel to an extent never seen in 

 those produced on loose stones or rocks, where shingle is hurried 

 along by a torrent, or by the waves on a sea-beach. 



As water is always flowing under some parts of a glacier, and much 

 melting and regulation are going on in different places, stones are lia- 

 ble to change their position, in which case a second set of strise and 

 furrows may be imprinted in a new direction, or another side of the 

 stone becomes, in its turn, flattened, striated, and polished. In like 

 manner the solid rock underneath the glacier may exhibit scratches 

 and grooves in more than one direction. The furrows will, most of 

 them, coincide with the general course of the valley ; but as the ice 

 in different seasons varies in quantity, the direction of its motion at 

 any given point is not uniform, so that the grooves and scratches will 

 also vary, one set often intersecting another. 



When a Swiss glacier, laden with mud and stones, descends so far as 

 to reach a region about 3500 feet above the level of the sea, the 

 warmth of the air is such that it melts rapidly in summer, and in spite 

 of the downward movement of the mass, it can advance no farther. 

 Its precise limits are variable from year to year, and still more so from 

 century to century ; one example being on record of a recession of 



