" Densities in the Earth's Crust." ?u ( .) 



to gravity at R must be equal to that of Q to gravity at R : 

 otherwise gravity at R would not have its normal value. 

 Hence the layers in the area P must be related to those in Q 

 in the manner proved. But it is not by any means necessary 

 that the number mat P should be the same as the number n 

 at Q ; and iu the particular case which I have shown to be 

 admissible at p. 244, there are at P on the land side two layers, 

 viz. the crust and the substratum, and at Q in the ocean four, 

 namely, the water, two in the crust, and the substratum. 



Mr. Blake further objects that the equations may be inde- 

 terminate. If they are so, it does not follow that no conclusions 

 can be drawn from them, because some o^ the unknown 

 quantities may lie within known limits, w r hich will give limiting- 

 values for the others ; and this is the method which I have 

 followed. But for the full understanding of the subject 

 reference must be made to chapter xvii. of my book and to 

 chapter xxvi. (Appendix). 



Seeing that a further investigation of my equations, sug- 

 gested by a letter from Mr. Jukes-Browne in ' Nature ' *, led 

 me to modify the conclusions given in chap, xvii., I think it 

 will be as well to state the results here, lest any of your 

 readers, not having seen the Appendix, should take the results 

 as stated in chap. xvii. as. final : — 



" (1) The suboceanic crust dips less deeply into the sub- 

 stratum than does the continental crust at the sea- 

 board. 

 " (2) The suboceanic crust is less dense in the lower than in 



the upper portion. 

 " (3) The lower portion of that crust is also less dense than 



the substratum beneath it. 

 " (4) The upper layer of the suboceanic crust is of high 

 density, and quite thin when compared with the whole 

 thickness. 

 " (5) The substratum beneath the ocean is less dense than 



beneath the seaboard. 

 " (6) The continental crust at the seaboard is of uniform 

 density throughout, or, if it does consist of two layers 

 of different density, one of them must be too thin to 

 sensibly affect the gravitational phenomena." 



* ' Nature/ vol. xli. p. 54 (1889). 



