542 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



mountains (of variously disturbed structure), and volcanoes (of 

 conical structure). The cognate landscapes of these forms of the 

 second order are familiar to us all, at least from illustrations. The 

 vegetation which tends to clothe them normally occupies a secondary, 

 dependent position in the hierarchy of nature. Moreover it is so 

 general in the case of continental plains and plateaux, and so bound 

 up with local features and conditions on mountains, etc., that it 

 would seem in the former instance to be pointless and in the latter 

 fruitless to attempt to describe it here. For plains and plateaux 

 generally have their rocks horizontally-bedded, with their deposits 

 or even igneous extrusions in flat layers, imposing an over-all 

 similarity of conditions and attendant vegetation ; contrastingly on 

 mountains the variability is so extreme as to defy brief description. 



Far more numerous and liable to be dependent upon vegetational 

 development are the landforms of the third order — the so-called 

 destructional ones. These are produced by the agents of erosion 

 working on the constructional forms, their characteristics being de- 

 termined in part by the erosional agent and in part by the con- 

 structional landform involved. Some of these features, such as river 

 valleys, are produced directly by erosion, the removal of material ; 

 others, such as river deltas, are the result of the importation 

 and deposition of sediment ; still others, such as natural bridges, 

 are residual, being left after the surrounding material has been re- 

 moved. Each and every one of these landforms tends to have its 

 own characteristic appearance and to contribute to the various types 

 of landscapes, which indeed are primarily made up of mixed land- 

 forms of the second and third orders. Secondarily, they are for the 

 most part veneered with vegetation, which of course varies according 

 to local climatic and other conditions, but nevetheless is often widely 

 comparable and sometimes actually characteristic of a particular 

 landform even throughout regions of very diff^erent climate. Thus 

 sand-dunes tend to be colonized by similar, coarse, binding Grasses 

 all the way from the Arctic to the tropics. 



As landscapes are primarily made up of various and variously 

 aggregated landforms, each of which indicates much of what has gone 

 on before and also of what may logically be expected to follow — 

 whether naturally or as a result of enlightened disturbance by Man — 

 their study is of both academic interest and applicational value. 

 This interest stems from the manner in which the earth scientist can, 

 from appropriate investigation of the landforms, often tell us much 

 regarding the past history of an area, while the value of such studies 



