96 Mr. J. Ball on the Formation of 



mainly lateral forces compressing the mass in the direction of the 

 meridian. To form a more accurate idea of the changes which 

 would result, let us take an imaginary meridional section 500 

 miles in length, and suppose that the least-resisting part of the 

 crust lies at the centre of the section, the rigidity increasing 

 gradually on either side. As we have already seen, the mechani- 

 cal effect which will be produced by the action of gravity (as- 

 sumed to be capable of overcoming the resistance of the mass) 

 will be to force each extremity of the section to approach the 

 centre by 75 yards. The first flexure having occurred at the 

 centre, the ordinary laws of the resistance of imperfectly elastic 

 and imperfectly rigid bodies lead to the formation of parallel 

 ridges with intermediate depressions extending on either side of 

 the first flexure within limits depending on the flexibility and 

 compressibility of the crust. We shall assume the limit at 50 

 miles on either side of the centre. The annexed figure will give 

 some idea of the nature of the vertical disturbances, the scale 

 being very greatly magnified to make them sensible to the eye, 

 and no attempt being made to exhibit the curvature of the 

 surface. 



Fig. 1. 



DCS 



E 1 



E 2 



D 3 C 3 E 3 



The dotted line A B shows the section of the original surface 

 before the period of disturbance, and A 1 B 1 that after subsidence, 

 with a series of ridges and depressions, highest at the centre C 1 , 

 and extending to D 1 and E 1 respectively 50 miles from C 1 . No 

 vertical disturbancewill have arisen in the portions A^ 1 and E^ 1 , 

 but a displacement has nevertheless been caused which deserves 

 attention. Having left out of view the changes going on in 

 neighbouring areas, we must assume that the subsidence from 

 A and B to A 1 and B 1 has been vertical ; but in that case the 

 portion of the section A D, which by hypothesis has not sensibly 

 contracted by cooling, and which has not suffered flexure, can 

 assume its new position A 1 D 1 only by the relative displacement 

 of D 1 60 yards nearer to C 1 , and a general displacement of the 

 whole section A 1 D 1 , which will gradually increase from A 1 where 

 it is nil, to the maximum at D 1 . The same displacement in the 

 opposite direction would occur in the subsidence ofEB to E^ 1 . 



Between the surface A B and a stratum lying at some depth, 

 probably considerable, where the rate of cooling would be 

 sensibly the same as that of the nucleus, there must be an 



