218 THE ANDES OF SOUTHERN PERU 



there may be uncovered a highly metamorphosed rim of resistant 

 sedimentary rock. 



The mere enumeration of these variable structural features is 

 sufficient to show how variable we should expect the associated 

 land forms to be. Were the forms of small extent, or had they 

 but slight distinction upon comparison with other erosional ef- 

 fects, they would be of little concern. They are, on the contrary, 

 very extensively developed; they affect large numbers of lofty 

 mountain ranges besides still larger areas of old land masses sub- 

 jected to extensive and deep erosion, thus laying bare many batho- 

 liths long concealed by a thick sedimentary roof. 



The differences between intruded and country rock dependent 

 upon these diversified conditions of occurrence are increased or 

 diminished according to the history of the region after batholithic 

 invasion takes place. Eegional metamorphism may subsequently 

 induce new structures or minimize the effects of the old. Joint 

 systems may be developed, the planes widely spaced in one group 

 of rocks giving rise to monolithic masses very resistant to the 

 agents of weathering, while those of an adjacent group may be so 

 closely spaced as greatly to hasten the rate of denudation. There 

 may be developed so great a degree of schistosity in one rock as 

 to give rise (with vigorous erosion) to a serrate topography; on 

 the other hand the forms developed on the rocks of a batholith 

 may be massive and coarse-textured. 



To these diversifying conditions may be added many others 

 involving a large part of the field of dynamic geology. It will 

 perhaps suffice to mention two others: the stage of erosion and 

 the special features related to climate. If a given intrusion has 

 been accompanied by an important amount of uplift or marginal 

 compression, vigorous erosion may follow, whereupon a chance 

 will be offered for the development of the greatest contrast in the 

 degree of boldness of topographic forms developed upon rocks of 

 unequal resistance. Ultimately these contrasts will diminish in 

 intensity, as in the case of all regional differences of relief, with 

 progress toward the end of the normal cycle of erosion. If pene- 

 planation ensue, only feeble topographic differences may mark 



