278 B. A. Daly — Mechanics of Igneous Intrusion. 



Influence of plutonic pressures on rock density. — Before 

 drawing any conclusions concerning the possibility of the flota- 

 tion of foreign blocks of solid rock in a plutonic magma, it is 

 clear that a preliminary and difficult 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 differ- 

 ences 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 mag- 

 mas need not affect the present argument except in a favor- 

 able way.* Since the temperatures of a block and its enclos- 

 ing 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 compression 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 

 irrespective 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 compounds can throw 

 light on the behavior of silicate magmas in other respects than 

 that cited in the foregoing quotation. This important deduc- 

 tion is corroborated by the proved similarity of silicates and 

 carbon compounds, in (a) the linear relation of expansion to 

 increment of temperature in the solid form of each substance, 

 in (b) the linear relation of expansion to increment of tempera- 

 ture in the liquid form of each substance, and in (c) the sudden 

 leap in volumetric increment in the act of melting at any 

 temperature.;); 



Barus further indicates that solid naphthalene is comparable 

 in compressibility with the liquid form of the same substance. § 



*Rep. on the Geol. of the Henry Mts., p. 76 (1877). 



fPhil. Mag., xxxv, 306 (1893), andU. S. Geol. Surv., Bull. 103, p. 55(1893) ; 

 cf. this Journal, xxxviii, 407, 1889, and xlvi, 141 (1893). 



% Barus and others have shown that even the exceptional type of fusion 

 represented by ice, becomes the normal type as a not high temperature is 

 reached under a condition of great pressure. This Journal, xli, 326 (1891). 



§ This Journal, xlii, 140 (1891). 



