292 CALIFORNIA ACADEMY OF SCIENCES. [Proc. 3D Ser. 



by undermining, and is, therefore, a wall of recession, results of profound 

 importance are to be looked for as an outcome of head-stream erosion. 

 Obviously, for example, recession here may carry the amphitheater rim 

 across the divides, and thus cut through the divide crest. 



In another portion of the statement, Mr. Johnson writes, 

 in speaking of the steps or terraces in the canyons: — 



Jointing may lend itself incidently to their development, or it may confuse 

 it. Commonly its influence would be unimportant, but the most clearly typi- 

 cal development is to be found where for considerable stretches the rocks 

 are massive and have no joints whatsoever. 



In all places where the steps above referred to are to be 

 found, my own observation is that there exists a horizontal 

 joint structure, although the joints may be hundreds of feet 

 apart, and consequently few joints, or sometimes no joints 

 at all, will appear on the escarpment of any given step. 

 For example, at Nevada and Vernal falls, where the step 

 structure is finely developed, the horizontal joints are very 

 evident, but there are none on the escarpment of the Ver- 

 nal Falls. 



From the foregoing, it will appear that glaciers may : 

 First, modify the basins in which they are born, so as to 

 form steep walled cirques; 



Second, modify the shape of the canyon down which 

 they flow ; 



Third, excavate under advantageous circumstances basins 

 in the underlying rocks. 



That they form the gigantic steps before described as 

 occurring in glaciated canyons is improbable, although they 

 enlarge them. Such steps occur along streams below the 

 limit of glaciation, where the rocks are intersected by hori- 

 zontal joints; and as stated above, in all those cases which 

 have come under my observation in the glaciated region of 

 the Sierra, horizontal joints are found in the granite wherever 

 the steps have formed. The theory that great canyons or 

 even considerable ravines are formed by the gouging action 

 of ice does not seem supported by the evidence. The con- 

 clusion may thence be drawn that the canyons of the Sierra 

 Nevada, like most other canyons the world over, were formed 

 in the main by river erosion. 



