Eev. 0. Fishei — Formation of Mountains. 61 



symbols, and to the paragraph at p. 257, " If however," et seq^., for 

 the proof of the equation, I have shown that 

 kle = S {a) - ^ (b). 



I now extend the inquiry to any area of the surface of length I and 

 width w, and the equation then becomes 



2 khve = S (A) — ^ (B), 

 where ^ (A) and ^ (B) are the volumes of the elevations above, 

 and of the depressions below, the " datum level." 



The whole surface of the globe being nest taken into account, the 

 relation becomes, 



Area of the Globe x 2 ke = :S (A) - ^ (B). 



It is important to understand what is meant by the "datum level." 

 It is an imaginary surface, which occupies the position which the 

 surface of the crust would occupy at the present time, if it had been 

 perfectly compressible, so that no corrugations would have been 

 formed in it by lateral compression. For it would in that case have 

 become simply more dense, without being disturbed in position. 



The above relation assumes that the elevations and depressions, 

 out of which the inequalities of the earth's surface have arisen, are 

 due to lateral pressure, and that such pressure has acted everywhere 

 parallel to the surface. Hence it is applicable to the earth's surface, 

 ■although that is not strictly regular in its general form, and may 

 contain local elevations and depressions affecting its mean figure, — • 

 that is, its mean figure as uninfluenced by lateral compression. For 

 these inequalities, though of small amount as compared with the di- 

 mensions of the globe, may be large in comparison with the quantities 

 of which we have to take cognizance in this investigation. Its truth 

 in no way depends upon the arrangement of the disturbed rocks, nor 

 upon the time at which successive movements have taken place, 

 nor upon the alternate elevations and depressions which have at 

 different times affected any given region. It includes every effect of 

 subsequent denudation, from whatever cause, and to whatever amount. 

 In short, it is perfectly general, so long as it is strictly interpreted. 

 But it does not take account of elevations or depressions of 

 regions of the surface arising from imequal contraction in a radial 

 direction, if their result should be to cause a defect of parallelism 

 between the datum level and the surface of the ocean, to which all 

 our measurements must be in practice referred. However, it does 

 not necessarily follow that contractions in the radial direction will 

 cause depressions in the ocean bed accompanied with a corresponding 

 increased depth of water. For instance, the defect from a true 

 circular form in the equator affects the surface of the ocean, to which 

 the measurements of geodesy are always referred, so that we do not 

 get an additional mile depth of ocean at the extremity of the shorter 

 radius. 



Assuming then that the continents have been shaped out of the 

 master elevations, and that the oceans indicate the positions of the 

 master depressions, and that both are ultimately due to lateral 

 pressure, I have estimated the value of 2ke for the whole globe 

 upon the following data : — 



