262 



Mr. J. Buchanan on 



portion of the torsion-curves would be part of a conic whose 

 focus is the centre of the concentric corrugations. 



Fi ? . 1. 



~^L 



S 



\/ 



^c 



The general resemblance between the curves in fig. 1 and 

 the torsion-curves drawn by Dr. M. M. Ogilvie for the Peri- 

 Adriatic area of subsidence is evident (Quart. Journ. Geol. 

 Sac. Aug. 1899, p. 630). 



It may be pointed out also that since an area of subsidence, 

 such a trough as the Adriatic for instance, is an area of 

 weakness, the contraction of the earth's crust due to secular 

 cooling would tend to move the torsion-curves more and more 

 closely towards this region as time goes on. The average 

 direction of motion at each point will be the direction of the 

 force of compression at that point. The torsion-curves will 

 on the average arrange themselves at right angles to these 

 forces of compression. Now, the curves which are orthogonal 

 to a system of confocal conies are themselves conies confocal 

 with that system. Hence, if the torsion-curves at any time 



