348 FOCUS OF ELEVATION. 



movement affected a large breadth of country from the Isle of Wight 

 to beyond the Thames valley, and is parallel to the upheaval of the 

 Alps, and the axis of elevation of central Europe of which it is a 

 consequence. 



Elevation the Consequence of Convulsion. That the effect of 

 convulsions has been, generally, to raise the convulsed area, will 

 appear evident by considering what is the focus of the disturbance 

 and the direction of its energy. The mountain chains and groups are 

 most certainly the foci of the disturbing forces ; for as we pass to- 

 wards them, from all sides the number and intensity of the dislocations 

 continually increase, and the inclination and contortion of the strata 

 grow more and more violent. The direction of the disturbing force 

 is clearly seen to be horizontal, while its effects are vertical or nearly 

 so, and thrust the folded masses outwards from the central regions of 

 the earth. It is like an "expansive force, which employed its principal 

 efforts along certain lines and about certain centres, breaking and 

 bending the strata in the highest degree, but also lifting them up 011 

 all sides around. Although the Mediterranean lies between the Atlas 

 chain and the Alps, the elevation of mountain chains and groups was 

 generally unaccompanied by any neighbouring violent depressions, 

 because the upheaval is only a part of a predominant upward bending 

 of the earthy crust. The inclination of the strata from mountain 

 chains for the most part gradually subsides to a gentle slope, and 

 finally vanishes in nearly horizontal planes. In the mountain chain 

 itself various and suddenly reversed dips may be met with corre- 

 sponding to the violence of the disruption, but by careful study the 

 general tendency of the convulsion may be clearly deciphered. 



The same data will not, however, by any means give us right to 

 conclude that the mountains so brought into existence were raised 

 above the sea, because, though we may know the absolute height of 

 the vertical movement, this will avail us nothing in our ignorance of 

 the original depth of the water. We must see whether mountains 

 bear on any part of their surface traces of those later marine deposits 

 which spread around their bases ; if they do, we may be sure they 

 were not elevated above the sea till after the date of these strata ; and 

 the Alps, for instance, bear upon their crests portions of oolitic, 

 cretaceous, and tertiary strata, and are thus proved to be of more 

 recent elevation than the geological age of the strata uplifted. If the 

 newer marine strata around their bases have been deposited horizontally 

 against the slopes of the mountains, we are entitled to believe that 

 the mountains had been previously reared above the sea. This con- 

 clusion, however, it must be always borne in mind, does not inform 

 us correctly concerning the height they were reared above the sea, 

 but leaves us to infer that they have since partaken of another move- 

 ment by which the newer strata have been placed at their present 

 elevation. 



