1871.] 



BOMTET FORMATION OF " CIRQUES. 



323 



high up in the limestone cliffs above the Aa-vaUey, on the left bank, 

 a short distance below Engelberg. Here six or seven small streams 

 issue from the rock, and have worked out a hollow only a few 

 dozen yards wide, yet of the true cirque type. 



Fig. 3. — Small Cirque near Engelberg. 





A. Limestone Clifik 

 streams break. 



B. Shaly bank with some trees &c., out of which the 

 C. Limestone cliff. D. Cirque. «»— *- Cascade. 



The conditions, therefore, most favourable to the formation of a 

 cirque are : — 



(1) Upland glens, combes or terraces, so shaped as to give rise 

 to and to maintain many small streams. 



(2) Strata, moderately horizontal, over which these streams fall, 

 which, by their constitution, yield considerably to the other forms 

 of meteoric denudation. 



(3) These strata must nevertheless allow of the formation of 

 cliffs ; and thus perhaps the most favourable structure is thick beds 

 of limestone, with occasional alternating bands of softer rock. 



Probably some favourable configuration of the ground must be 

 also assumed at the beginning : if a glen ended in a cliff, it would 

 doubtless be more readily cut into a cirque; but whether this 

 is always needed, or what cause has made the cUff, I do not now 

 attempt to investigate. I venture to submit that I have proved 

 that glaciers cannot have produced the cirques, and that (since 

 these cirques cannot be postglacial) they have not, to any great 

 extent, excavated the Alpine vaUeys. To assign to each agent its 

 due share in the task of erosion in the Alps (and in all other moun- 

 tain-regions that I have seen) appears too complicated a problem 



