APPENDIX. 205 



aid of observers with the discussion of some simple hypothetical conditions of contorting 



action. 



There is an important agent in the formation of the earth's orography, which has 



not vet been mentioned, hut of which it is most important to 

 Denudation. 



indicate the action to the general observer. Denudation is a 



directly antagonistic power to elevatory forces, its ultimate action tending to remove all 



inequalities of surface, but in doing so its immediate result is the production of the most 



intricate irregularities. A study of the existing state of any portion of the earth's surface 



will show that denudation is in fact more a hill-maker than a hill-destroyer ; bv far the 



greater number of what we call hills are its immediate production. In what I say here, I, 



of course, allude to sub-aerial, pluvial denudation, by rain and rivers. Oceanic denudation 



may perform a greater amount of work in abrading and transporting matter ; it may 



remove many a thick covering from a slowly rising area ; it may cut out coast-lines, more 



or less indented, which subsequently become inland hills ; it, no doubt, too, leaves shallow 



lines of hollow by which subsequent drainage lines are initially determined ; but, as a rule, 



and as compared with pluvial denudation, it is purely a levelling agent ; it carries, away 



wholesale where the other agency would work out mountain systems on its own principles. 



The normal results, such as would be produced tinder homogeneous conditions, of these 



two agencies of hill-formation are very different. The tendency of 

 Its results compared with , , „ . , j t i> i 



those of elevation. subterranean forces is to produce lines or zones of elevation, more 



or less longitudinal or concentric. The result of pluvial action 



upon a level homogeneous mass would be to produce a symmetrical system of hills having a 



central longitudinal axis with regular primary offshoots, and from these again minor spurs. 



In this mode of formation the secondary or minor resultant ridges are as characteristically 



transverse as in the other they are longitudinal. It has indeed been advanced as a canon 



in geological dynamics that all drainage is originally transverse ; the longitudinal valleys 



being completed by the gradual encroachment upon each other, and the ultimate union, of 



what were at first but longitudinal (with reference to the mountain axis) feeders of the 



primary transverse streams : each primary stream so absorbed becomes an affluent of the 



united longitudinal feeders which have now become the main line of drainage. Thus the 



degree in which either form is stamped upon any system of ridges, or on parts of that 



system, may serve as an indication of the influence that either agency has exerted in modelling 



the actual orography. 



In all discussions upon mountain ranges and their directions, in every attempt to define 



ridges and lines of elevation as an element of terrestrial 



Importance of distinguishing pbvsics, it is only in so far as these ranges or ridges belong to 

 the two. ° ° 



the first (the subterranean) order of phenomena that any interest 



attaches to them. On any other ground there would be really little or nothing of primary 



interest to discuss, beyond the mere topographical or physico-geographical feature. It 



becomes then of essential importance to distinguish the effects of these two agencies ; it is 



utterly confounding the subject to set down as a ridge or a system of ridges, and without 



special mention, a mere series of contiguous elevations forming an irregular watershed, and 



such as, it is easy to understand, must result under certain conditions from pluvial action 



alone. For example, unless geologists are forewarned that their ideas are not taken into 



