the Earth's Crust. 



345 



the crust, the under surface of which is supposed to be uniform. 

 This segment of the crust is not only treated as a nearly flat 

 plate band, but great inequalities of thickness are assumed ; and 

 the problem is thus invested with circumstances from which it 

 readily follows that a very thick crust becomes necessary in 

 order to compensate such remarkably weakening conditions. As 

 we know that the first of these conditions is decidedly incorrect, 

 and that the other at least is wholly unsupported by facts, we 

 cannot be expected to admit the conclusion to which they would 

 lead.'' (P. 25.) 



The numbers in brackets, I have introduced for convenience 

 of reference in making the few following remarks : — 



[1] The portion of the crust of which I, in the first instance, 

 calculated the conditions of equilibrium, is 100 miles long ; that 

 is, less than 1° 30', and not 24°, as stated in this extract. (It is 

 AM in the diagram, p. 331, in your May Number.) The cur- 

 vature of this is insensible. In a second example I applied the 

 same principles to an arc of 1500 miles, or about 21° 30', run- 

 ning south from Cape Comorin. But in this case, as the ten- 

 dency is not for the crust to sink, but to be broken upwards by 

 the pressure of the lava, the arched form of the crust can offer 

 no resistance to fracture. 



[2] There are one or two expressions in this latter part of 

 Professor Hennessy's remarks which I do not quite understand. 

 I fear I have not been sufficiently clear in my explanation. I 

 will therefore give the calculation again, and with more detail, 

 and with as near an approach as I can to the actual geography 

 of the parts which I bring under consideration. 



1 . Colonel R. Strachey, in his account of the Himalaya Moun- 

 tains (now in the press), gives an outline map of the vast plateau 

 of the Range, with the northern and southern slopes. By the 

 help of this map the accompanying diagram is constructed. (In 

 drawing it, I have transported myself to England and supposed 

 myself viewing this country from the west.) 



A is the station Dehra (lat. 39° 10', long. 78° 6'), about 2000 



