288 FKAGMENTS OF SCIENCE. 



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 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 formation 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 produced by small streams the 

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

 Right 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 Via 

 Mala throws light upon the whole. Near Bergiin, in 

 the valley of the Albula, there is also a little Via Mj?la, 



