The Glacial Theory of Prof. Agassiz. 35^ 



while small stones often sink into cavities, large ones seem hoisted on 

 pedestals. Masses of all kinds tend towards the sides of the glacier, 

 and many of these huge blocks are found scattered along the flanks of 

 the Alpine valleys, some having remained there, stranded as it were. 

 Others are found in the middle, far from existing ice, and were proba- 

 bly left there when the glacier disappeared. We have thus an expla- 

 nation of the erratic blocks so common in this country, when these do 

 not come from very distant stations. Being stranded by their greater 

 weight, while the smaller matter moved onward, or left sticking on the 

 soil in consequence of the final fusion of the ice, we can understand why 

 they are often found perched on the sides of steep declivities. 



Blocs perches, so named for the reason just given, are sometimes 

 found in very singular situations. 



Let a, figure 6, be the surface of the glacier, r the top of a project- 

 ing rock in situ. The ice has the block h floating on it ; it encom- 

 passes the fixed rock nearly on a level with its summit, and in travelling 

 downward strands the block upon it. The block may be stranded on 

 the very summit, as c. -p- g p- ^ 



Supposing the glacier af- 

 terwards to disappear, 

 here we would have an 

 angular block perched on 

 an isolated hill, or as Agas- 

 siz terms it, a pyramid, ^ ' * ■ ' "" ~ -^ 

 with a steep declivity below it, and we would be puzzled to conceive 

 by what agent it was planted in so singular a situation. 



Figure 7 represents erratic blocks in a different situation, but quite 

 as singular, r is a projecting fixed rock, rising considerably above the 

 glacier a a ; the reflection of the sun's heat from its surface melts a 

 portion of the ice, and forms a cup-shaped cavity round it. Into this 

 cavity blocks of various sizes fall by their weight from the surface of 

 the ice as it glides onward, and settle on the flanks of the hillock. Sup- 

 posing the glacier to disappear, this conical rock would have a ring of 

 stones like a coronet encircling^ its summit, and we would be apt to 

 wonder at the mysterious agency which brought them there, and left 

 the lower pai'ts of the hill destitute of them. Agassiz names various 

 isolated rocks amidst the Alpine glaciers with such circles of stones 

 round them, or with single blocks stuck upon them, as in figure 6. 

 The same phenomena reappear on Mount Jura, where no glaciers now 

 exist. 



Creux and Lapiaz. — On the sides of the Swiss valleys, round holes, 

 such as cascades make, are sometimes found in the rock ; but in places 

 remote from running waters, and where the form of the surface will 



