744 



DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



Beade's coefficients enable us to calculate the approximate changes in specific 

 gravity undergone by blocks of stratified and schistose rocks (common country- 

 rocks about batholiths), as these rocks (arbitrarily regarded as still solid) 

 assume the temperature of very hot magma (at 1300° C.) in which they are 

 immersed (Table LIT.) :— 



TABLE LII. 



> 



Range of sp. gr. 

 at 20°C. 



2-60-2-80 

 275-310 



220-2-75 

 240-2-80 

 2-65-2-80 



Range of sp. gr. 

 at 1300°C. (solid). 





252— 271 





267— 300 





2-13-2 67 

 232— 271 





2-57-2-71 



Influence of Plutonic Pressures on Rock Density. — Before drawing any 

 conclusions concerning the possibility of the flotation of foreign blocks of solid 

 rock in a plutonic magma, it is clear that a preliminary stage of our 

 inquiry must be passed. What influence has pressure at great depths on the 

 relative densities of solid blocks and of the liquid magma in which they are 

 immersed? One can hardly doubt that water and mineralizers in depth would 

 increase such differences as those calculated for one atmosphere of pressure 

 and 1400° C; so that Gilbert's conclusion as to the difficulty of determining 

 the densities of hydrothermally molten magmas need not affect the present 

 argument except in a favourable way.* Since the temperatures of a block and 

 its enclosing magma are practically identical, the final step in deciding on their 

 relative densities in depth is taken, if it can be shown what is the relative com- 

 pression suffered by the solid and liquid. 



Again we must have recourse to the valuable experiments of Barus as those, 

 of any known to the writer, most nearly related to the problem at issue. He 

 concludes, as a net result of his investigations, that ' the relation of the melting- 

 point to pressure in case of the normal type of fusion is nearly constant irre- 

 spective of the substance operated on. . . And in the measure in which this 

 is nearly true on passing from the carbon compounds to the thoroughly different 

 silicon compounds, is it more probably true for the same substance changed only 

 as to temperature and pressure. In other words, the relation of melting-point 

 to pressure is presumably linear.'f Accepting his inferences as sound, the fact 

 remains that his experiments on thymol, naphthalene, and other carbon com- 

 pounds can throw light on the behaviour of silicate magmas in other respects 

 than that cited in the foregoing quotation. This important deduction is cor- 



* G. K. Gilbert, Rep. on the Geol. of the Henry Mts., 1877, p. 76. 

 fPhil. Mag-., Vol. 35, 1893, p. 306, and U.S. Geol. Survey, Bull. 103, 1893, p. 55;cf. 

 Amer. Jour. Science, Vol. 38, 1889, p. 407, and Vol. 46, 1893, p. 141. 



