A. J. Jukes-Browne — Mellard Readers Mountain Building. 25 



In the first place I will deal with three objections which seem to 

 me to be capable of being partially answered : 



1. Mr. Fisher, reviewing the book in this Magazine/ criticizes 

 Mr. Keade's conception of the physical state of the material below 

 the crust : the wording of the passage quoted from Mr. Eeade's book 

 that this material is " solid by compression, but ready to flow one 

 way or other, as the pressure may be reduced or increased," is 

 certainly loose, but the context plainly shows Mr. Eeade's conception 

 of the material at about thirty miles depth to be that it is permanently 

 plastic, and only kept from liquefaction by the pressure of a thirty 

 mile crust above it. If the pressure were relieved, it would become 

 liquid ; if everywhere increased, it would become rigid ; but if the 

 plastic zone is deep, and a small area of the crust is weighted and 

 depressed, the plastic material beneath may be displaced (without 

 being made to flow as a liquid) and might have the effect of slightly 

 bulging up the crust around the depressed area. This is Mr. Eeade's 

 view, but his use of the word flow suggests the idea of the liquidity 

 which he did not mean to convey. 



2. Mr. Fisher finds a second difficulty in the fact that the trans- 

 ference of heat from the plastic magma to the depressed crust would 

 take place concurrently with the depression, " so that the swelling 

 up (by expansion) would begin at once." Mr. Eeade's own reply 

 to this (Phil. Mag. 1891, p. 491) does not seem very happy : his 

 own book (pp. 93 and 122) affords a better answer; for he says the 

 first accession of heat would be employed in lateral expansion, and 

 in folding and crushing the materials of the crust which underlay 

 the newly-deposited sediments. He thinks the expansion of the 

 lower layers of the weighted crust, being confined by the surrounding 

 tracts and by the weight of the upper layers, would develope internal 

 strains, and that much of the heat would be expended in doing work, 

 i.e. in developing a force which would ultimately produce foliation 

 by a process of forcible detrusion, crushing and repacking or refor- 

 mation of constituents, when at length it became easier for the 

 expansive force to lift the overlying mass of the crust rather than 

 to exercise further lateral compression. 



The above are Mr. Fisher's two principal objections to the theory 

 of upheaval by cubical expansion, and he afterwards says, " If the 

 two preliminary difficulties can be disposed of, the theory seems well 

 suited to explain the formation of elevated plateaux. But for pro- 

 ducing the intense corrugation, which characterizes most mountain 

 ranges, the amount of horizontal expansion which it afi'ords appears 

 inadequate." In this remark I quite agree with him for reasons 

 which will appear in the sequel. 



3. Before, however, we can accept Mr. Eeade's theory as a real 

 cause of upheaval, we have to reckon with Mr. Davison, who has 

 published what he regards as a fundamental objection to the theory.- 

 He writes, however, as if it was the accumulating sediment only 

 that received an accession of heat, and as if the crust below did not 

 partake in this accession. He says the heat which expands the 



1 Dec. III. Vol. IV. p. 229, 1887. ^ Geol. Mag. May, 1891, p. 211. 



