236 FKAGMENTS OF SCIENCE. 



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 

 fracture theory is, that the valleys themselves follow 

 the tracks of primeval fissures produced by the up- 

 heaval of the land, the cracks across the barriers re- 

 ferred to being in reality portions of the great cracks 

 which formed the valleys. Such an argument, how- 

 ever, 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, some- 

 times 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 formation 

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

 cribed 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 produced by small streams the 

 beds of which are dry for a large portion of the year. 

 Eight and left of the larger gorges such secondary 

 chasms are often found. The idea of time must, I 

 think, be more and more included in our reasonings on 

 these phenomena. Happily, the marks which the rivers 

 have, in most cases, left behind them, and which refer, 

 geologically considered, to actions of yesterday, give us 

 ground and courage to conceive what may be effected 

 in geologic periods. Thus the modern portion of the 



