DESCRIPTIVE GEOLOGY: GENERAL CONSIDERATIONS. 265 



or U. Tertiary zone, that its north-western neighbour the Nummulitic 

 zone is somewhat less backward, whilst the Slate zone, and the Crys- 

 talline and metamorphic zone, respectively show signs of a more 

 complete or prolonged denudation. 



Altogether then, taking the zones in order from A to D, that is to 

 say, from north-west to south-east, we find them to be in unison 

 as regards the amount of elevation, compression, and denudation 

 which they have suffered : zone A has been most elevated, most com- 

 pressed, and most denuded ; whilst as we travel south-east through 

 the other zones in order we find that they have successively been less 

 elevated, less compressed, and less denuded. 



There are two principal views which may be held as to the cause 

 Two ways of account- ° f the above three-fold progression ; we may 

 ing for the above either regard it as the joint result of a varying 

 intensity of the activities at work, that they 

 were more energetic in zone A than in zone B, and so on ; or, allowing 

 their intensity to have remained constant throughout the whole area, 

 we may account for the different progressive results by supposing 

 that the time during which those activities were at work varied 

 instead, a view of the matter which would range the zones in order 

 of age A > B > C > D. 



A third view compounded of the other two is of course possible 

 also. 



For the present we must leave the question an open one until we 

 have considered some other points which bear upon it. 



Turning to a consideration of the boundary faults which lie on 

 ^ the south side of each of the three more 



Remarks on the boun- 

 dary faults to the south northern zones of disturbance, we may note 

 of each zone. . 1f , . ,, 



specially about them : — 



(1) Their parallelism to the general strike of the adjacent forma- 



tions : a fact which marks them as being of the nature of 

 fold-faults, or highly dipping thrust-planes. 



(2) Their characters as being lines dividing one zone from ano- 



ther : thereby indicating how the extra elevation and 

 compression of each zone found relief by the packing up 



( 265 ) 



