MAPPING. 



through large parks, woods, moors, and marshes, where, 

 perhaps, no evidence whatever can be obtained. In 

 these cases the difficulty may be considerably lessened, 

 by first mapping for a good distance all round such ob- 

 scure areas. With the data and the ideas thereupon 

 thus gained, the lines may then be run by feature alone 

 with every chance of accuracy. 



Where there is but little or no change of feature, the 

 line being traced should be kept well up above where 

 it would at first seem to run for this reason, not only 

 is the debris of the upper rock constantly falling upon 

 the outcrop of the lower, but the soil also which properly 

 belongs to it is continually being washed down to a 

 lower level. This for some distance down gives a decep- 

 tive indication of the upper rock, where the lower only 

 would be found on making an excavation. 



The varying hardness and the dip of the rocks give 

 the form to the district in which they come to the sur- 

 face. For, precisely in inverse proportion to these con- 

 ditions, have the agents of denudation worn them away ; 

 or, to put the proposition in another form, the promi- 

 nent minor features of a district are in exact proportion 

 to the power its rocks possess of resisting denudation. 

 As dissimilar rocks thus make a change of feature 

 along their junction, a knowledge of the fact is, as we 

 have seen, of great service in drawing their lines of 

 boundary. 



But denudation will also obscure the minor features 

 that it has made, for the rain- wash which is the immedi- 

 ate result of sub-aerial erosion will lodge on projections 

 and fill up hollows. The eye will, however, by practice, 

 get accustomed to look and to make allowance for this 



