Wilson — On Contortions and Faults. 



207 



The piec>-s a, a, o, 0, are of the same 



cracks, passing riglit through the solid crust, taking place in a 

 variety of directions, and all the pieces, which are broader at the 

 surface than lower down, sinking further relatively to the rest. 



It will be seen at once by reference to a diagram, which I have 

 made on an exaggerated scale (see Fig 4). If the mass a b is elevated 

 so that it assumes the more curved i''?- 4. 



form c D, A B will crack; (a) will 

 rise, and {d) will rise, but the in- 

 creased space between (a) and (d) will 

 be occupied by the sliding down of 

 the pieces (6) and (c). Then since 

 these faults always take place when 

 the area is depressed, the rise will take 



place under the sea, and be extremely ti^eliom cT Dt^Ti t^JJ^f"^ 

 slow, and the marine denudation will a to b, 



soon level the surface, and obliterate all trace of faults. It is clear 

 that this explains the general law of faults given above ; and puts 

 contortions and faults in connection with one another. Faults then 

 are the inevitable result of the elevation of a curved surface. The 

 only point that needs further examination is this, whether the cause 

 assigned above is adequate to produce the observed amount of faults 

 and contortions. 



I have examined this question mathematically, and the following 

 are the results I have obtained on the supposition that a circular area 

 of the earth's surface whose diameter subtends an angle of 2 ^ at the 

 centre of the earth, is depressed, so as to maintain a spherical form, 

 (of course a portion of a larger sphere) to a depth of {a) miles in 

 the centre of the depressed region ; — that is on the supposition that 

 the arcs a b and c d, as in the diagram, are both circular. The cal- 

 culation only requires a little trigonometry, and may be relied on as 

 true within a few yards. 



Table of Compression, in Yards. 



For an arc of 2 0, depressed 



;to a depth a ; radius = 4,000 miles. 





= 5° 



e=w 



= 20° 



= 40° 



o = I mile 



121 



211 



354 



800 



o = 2 miles 



189 



408 



745 



1625 



o = 4 miles 



355 



788 



1584 



3214 



o=8 miles 



598 



1527 



3213 



8539 



linear distance 



across depressed 



area 



345 miles 



690 miles 



1380 miles 



2760 miles 



