14 t/. W. Judd — On Yolcanos, 



ing that the present surface of tlie eartli is subject to slow but 

 powerful movements, sometimes wide-spread in their operation, but 

 at others exceedingly local. The modes of reasoning by which 

 geologists have arrived at these conclusions concerning the move- 

 ments of the earth's surface are not less cogent and convincing than 

 those which they adduce in support of their views concerning the 

 effects of denuding agents. And it is quite possible to admit to their 

 fullest extent the important part played by atmospheric waste in the 

 moulding of the features of the earth's surface, without persistently 

 shutting our eyes to the effects of those subterranean forces, concern- 

 ing the operation of which we have equally convincing evidence. 



The only mode of escaping from this mode of reasoning is by 

 denying that local and differential movements, such as have so con- 

 stantly produced bending and fracture in the strata, are still at work 

 on the earth's crust ; or, as Mr. James Geikie appears to do,^ to as- 

 sume that they can produce no effects at the surface. That faults 

 do not produce "lines of cliffs " at the surface (except perhaps under 

 peculiar and exceptional conditions) we are ready to admit — for the 

 denuding forces are constantly at work masking and modifying the 

 effects of the subterranean ; and both are equally slow and all but 

 imperceptible in their modes of action during the limited periods of 

 human observation. But for the conversion of an ordinary river- 

 valley in part of its course into a lake-basin, it is by no means neces- 

 sary that any movement of so great and violent a character as to 

 produce a fault in the subjacent rocks should take place. Any one 

 who will examine the longitudinal section of a lake-basin accurately 

 drawn to scale, such, for instance, as the instructive examples given 

 by Professor Eamsay, must admit that an almost imperceptible 

 curvature of the strata, to the extent of two or three degrees only, 

 will suffice to produce even the deepest known lakes. 



That lines of flexure and fracture must have had much to do in 

 the oiiginal determination of the lines of drainage of a district, it is 

 impossible to doubt. And that periods of violent movement in a 

 district may have resulted in important modifications and vast altera- 

 tions in its system of drainage, few will hesitate to admit. Where 

 too, as in the case of Lough Neagh, the detailed mapping of the 

 district by a competent observer brings to light faults, the position, 

 effects, and age of which are exactly such as would result in the 

 surface movements necessary to produce the rock-basin in question, 

 we are surely justified in inferring a connexion between the two 

 sets of phenomena. But it by no means follows that where we are 

 unable to detect a fault crossing the line of valley or a synclinal fold 

 in its course, there subterranean movement could have had no part 

 in producing a lake-basin in it. The amount of vertical movement 

 necessary to originate even the deepest known lake-basins bears so 

 small a proportion to the length of the valleys in which they lie that 

 we do not hesitate to affirm that their effects upon the subjacent strata 

 could not, save under exceptionally favourable conditions, be de- 

 tected by the most experienced geological surveyors. 



It is only by those who ignore altogether the operation of subter- 

 1 " The Great Ice Age," page 289. 



