BY E. M. JOHNSTON, F.L.S. 55 



due to circumferential contraction the rocks may be rendered 

 fluid by relief of pressure. Such melting, he adds, would be 

 accompanied by expansion manifesting itself at the surface by 

 an uplift. By the expansion of the melting under] ayers tensile 

 stress in the overlying strata would be called into play, and 

 this would throw those strata into a state of tensile strain, thus 

 giving origin to normal faults to the gradual gaping of mineral 

 veins and dykes, into which the molten matter would be 

 injected by the expansive force. Without denying, therefore, 

 the influence of secular refrigeration, he suggests that we have 

 on this hypothesis an efficient primary cause of volcanic 

 eruptions. Commenting upon the differences of opinion as to 

 the level of the stress he points out that according to Mr. 

 Davison it lies five miles deep ; according to Mr. Mellard 

 B-eade it is taken at one mile ; while Mr. 0. Fisher would 

 reduce it to less than a mile ; and accordingly Prof. Morgan 

 urges caution as to the use of precise imposing mathematical 

 calculations based upon arbitrarily selected data where it 

 concerns problems " concerning which the most noteworthy 

 feature is our profound ignorance." 



Mr. Danvees-Powee's Befeeence to the Supposed 

 Influence of the Peessuee of Ocean Watees upon 



THE FOEMATION OF PaEALLEL MOUNTAIN CHAINS. 



In regard to this hypothesis I must confess that to me it 

 seems to be a most extraordinary one. Mr. Danvers-Power 

 does not give us the slightest indication to help us to conceive 

 how, of all agencies that may be concerned in the dynamics of 

 mountain making, " the presence of the ocean is the greatest." 

 It is hardly necessary to remind us how great is the influence 

 of water in the work of denudation, and in the redistribution 

 of wasted rock sediments over lower levels of sea and land ; 

 but how the gravity or even surface movements of ocean 

 waters extending over wide ocean areas can concentrate their 

 force of gravity or pressure laterally by thrust upon the 

 margins of continents is a nice puzzle to any physicist. Surely 

 it must have occurred to Mr. Power that the vertical radii of 

 a column of rock is from £§ to 3.4 times the specific gravity 

 of an equal column of water ; that the radii of a continental 

 area are much greater than the radii over the more depressed 

 ocean areas, capped as they are with the light element water ; 

 that the pressure of a fluid upon any of its limiting surfaces is 

 altogether independent of its quantity (known as the 

 hydrostatic paradox) ; that is — the total pressure of water, 

 (still) against and perpendicular to any surface is equal to the 

 weight of a uniform column of water, the area of whose cross- 

 section parallel to its base is everywhere equal to the area of 



