250 FRAGMENTS OF SCIENCE 



sides of the great chasm that instruction was needed, and 

 from its edge nothing to satisfy me could be seen. 1 

 therefore stripped and waded into the river until a point 

 was reached which commanded an excellent view of both 

 sides of the gorge. The water was cutting cold, but I was 

 repaid. Below me on the left-hand side was a jutting cliff 

 which bore the thrust of the river and caused the Aar to 

 swerve from its direct course. From top to bottom this 

 cliff was polished, rounded, and scooped. There was no 

 room for doubt. The river which now runs so deeply 

 down had once been above. It has been the delver of its 

 own channel through the barrier of the Kirchet. 



But the broad view taken by the advocates of the fract- 

 ure theory is, that the valleys themselves follow the tracks 

 of primeval fissures produced by the upheaval of the land, 

 the cracks across the barriers referred to being in reality 

 portions of the great cracks which formed the valleys. 

 Such an argument, however, would virtually concede the 

 theory of erosion as applied to the valleys of the Alps. 

 The narrow gorges, often not more than twenty or thirty 

 feet across, sometimes even narrower, frequently occur at 

 the bottom of broad valleys. Such fissures might enter 

 into the list of accidents which gave direction to the real 

 erosive agents which scooped the valley out; but the for- 

 mation of the valley, as it now exists, could no more be 

 ascribed to such cracks than the motion of a railway train 

 could be ascribed to the finger of the engineer which turns 

 on the steam. 



These deep gorges occur, I believe, for the most part in 

 limestone strata; and the effects which the merest driblet 

 of water can produce on limestone are quite astonishing. 

 It is not uncommon to meet chasms of considerable depth 



