TOPOGRAPHY 3OI 



to correlate the accumulations of sediments with the denuding 

 processes which furnished the material. 



The topography of any land area may be considered as the 

 outcome of a struggle between two opposing sets of agencies : 

 (1) those which tend to upheave the region and thus increase its 

 elevation ; (2) those which tend to cut down the land to the level 

 of the sea. The latter comprise the agencies of denudation, or 

 degradation, while the former are the diastrophic agencies, or simply 

 called dias trophism. Two kinds or manifestations of diastrophism 

 may be distinguished: (a) Epeirogenic (from the Greek epeiros, con- 

 tinent), the broad uplifts or depressions of areas, not necessarily 

 accompanied by folding or tilting of the strata ; (o) Orogenic 

 (from the Greek oros, mountain), the upheaval of relatively 

 narrow belts of land, caused by the lateral compression of the 

 strata. It is not yet known whether these two processes differ 

 in principle, or whether they are merely different manifestations 

 of the same agencies. Sometimes, indeed, the degrading and dia- 

 strophic agencies cooperate, as when the land is depressed instead 

 of upheaved, but this is not the more common condition. 



The details of topography are, in large degree, controlled by 

 still a third class of factors, which, however, are passive rather 

 than active ; namely, the character, arrangement, and attitude of 

 the rock masses. A partially degraded region in which the rocks 

 are homogeneous will have a very different kind of relief from one 

 in which the rocks are heterogeneous and differ materially in their 

 powers of resistance to the denuding agents. A region of hori- 

 zontal strata will give rise to very different topographical forms 

 from those which are developed in areas of folded or tilted strata. 

 We must further distinguish between regions whose topography is, 

 in the main, due to constructive processes from those in which 

 denudation has prevailed. Examples of such constructive forms 

 are volcanic mountains, and plains or plateaus formed by widely 

 extended lava flows, plains newly deserted by the sea and due to 

 sedimentation, alluvial plains of rivers, and the mounds, ridges, 

 or sheets of drift spread out by the action of glaciers and of the 

 waters derived from their melting. 



