Alpine Valleys and Alpine Lakes. 



97 



intermediate condition of portions of the crust which well de- 

 serves notice. The lines A 2 B 2 and A 3 B 3 represent the condition 

 of some portions of the crust where the amount of cooling during 

 the assumed period has caused a contraction of 50 yards and 

 100 yards respectively in the length of the portion lying between 

 the verticals A B 3 and B B 3 . In each case the insufficient con- 

 traction of those portions of the crust must be supplemented by 

 some flexure and disturbance of the strata, but the deeper we 

 descend, and the more the rate of cooling has approached to 

 that of the interior, the less disturbance will have arisen. 



In the illustration here proposed, it will be seen that if the 

 zone of least resistance had happened to lie due E. and W., 

 every meridional section across the same area would show the 

 point of maximum flexure at the centre, and we should have a 

 single mountain chain running due E. and W. across the middle 

 of the area ; but if the zone of least resistance should happen to 

 lie in some direction not per- 

 pendicular to the meridian, we 

 should have a series of parallel 

 ridges arranged as in the an- 

 nexed figure, where M N repre- 

 sents the zone of least resistance, 

 and the broad black lines the 

 axes of as many systems of 

 parallel ridges. The fact that 

 such a disposition of mountain 

 ranges frequently occurs, is 

 familiar to all who have studied 

 the orography of the Alps and 

 other high mountain districts. 



In the incalculable lapse of geological time during which the 

 same causes must have continued to cause disturbances of the 

 outer crust of the earth, but under varied conditions of resist- 

 ance, it is clear, on the one hand, that the direction in which the 

 force of compression would act on a given portion of the surface 

 would be liable to continued but slow variation, and on the other, 

 that there must be an interdependence between the disturbances 

 arising in adjoining regions, even though these should cover 

 very large areas. As a general rule it seems inevitable that a 

 mountain district once formed would become for the surrounding 

 regions a zone of least resistance, which would be liable to 

 undergo new flexures according as pressure came to operate in 

 new directions, and that we ought to find the traces of such 

 transverse compression in the formation of parallel ridges lying 

 at various angles to the main lines of flexure, and at the same 

 time that these secondary ridges should be far less regular than 



Phil. Mag. S. 4. Vol. 25. No. 166. Feb. 1863. H 



