2l8 



Sierra Club Bulletin. 



plates are comparatively unaffected. The removal of the 

 rock is essentially through a process of peeling. One layer 

 at a time is carried away, and the surface at each stage 

 coincides approximately with one of the partings. 



Whatever the cause of the dilatation producing the 

 partings, they are formed in succession from without in- 

 ward. For each one the determining strains are themselves 

 conditioned not only by the form of the outer surface, but 

 by the form of the last-made parting. Parallelism is not 

 perfect, but approximate, and the departures from strict 

 parallelism are of such nature as to reduce or omit 

 angles and other features of irregularity. The inner 

 partings reflect only the general features of the external 

 sculpture. As peeling progresses and the zone of com- 

 petent strain moves inward, the outer surfaces are suc- 

 cessively more and more simple in contour, and the newly 

 developed partings are endowed with still greater sim- 

 plicity. 



Opposed to the rounding process is corrasion. The 

 attrition of a detritus-armed stream or glacier saws 

 through the rock-plates with little regard for the presence 

 or absence of partings. By so doing it creates discordant 

 elements of topography and modifies the conditions under 

 which the expansive strains are developed. In the Sierra 

 the effects of glacial corrasion are at present conspicuous. 

 By the corrasion of the Tenaya trough the base of Half- 

 Dome was sapped, so that a part was sheared off by grav- 

 ity, producing a vertical flat face (figure i), in which the 

 structureless nucleus was exposed. In this face the 

 "dome structure" was developed, but, being conditioned by 

 a plane outer surface, the new partings are plane (except 

 at the edges), and thus simulate ordinary plane joints. 



