276 THE ICE AGE IN NORTH AMERICA. 



gists might be cited here, but it is not my intention to review 

 the entire literature of the subject. 



Sufficient observations have been recorded to show not 

 only that cirques are of nearly world-wide distribution, but 

 that they are confined to glaciated regions, and are not found 

 in mountains where undisputed records of glacial action are 

 absent. This in itself is good evidence that they have resulted 

 from glacial erosion. The same conclusion is indicated by the 

 fact that as a rule they open northward — that is,, they occupy 

 positions where glaciers first appear when a lowering of tem- 

 perature renders their existence possible, and where they lin- 

 ger longest when the climate ameliorates. It thus seems un- 

 necessary to discuss in the present paper the various hypothe- 

 ses which refer their origin to water erosion, crater elevation, 

 etc. 



The descriptions presented in the essays we have cited, 

 together with the observations of the writer, show that the 

 cirques of the high Sierra are typical of their class, and present 

 all the features to be seen in other similar regions. It is thus 

 rendered evident that, if we can arrive at an acceptable expla- 

 nation of their origin, it should explain the like phenomena 

 in other regions as well. The writer has no mature theory to 

 offer, but hopes to contribute something toward the desired 

 end. 



It is usually difficult to draw a definite line between a 

 glacial cirque and the canon leading from it. One is a contin- 

 uation of the other. It is evident, also, that the walls inclos- 

 ing a cirque have many features in common with the scarps so 

 frequent in glaciated caflons. When the cirques themselves 

 are terraced, this analogy is rendered still more complete. The 

 writer's studies in the high Sierra and elsewhere have led to 

 the conclusion that such scarps and cirques result mainly but 

 not wholly from glacial action. The initiation of the process, 

 at least in the high Sierra, as in the case of many glacial 

 canons, must have been by subaerial erosion. 



Lorange's observations show that when a neve fills a 

 cirque it is capable of removing blocks of rock from the inclos- 

 ing walls. The fact that these walls are rough and angular, 

 instead of smoothly polished, is proof that there is but little 



