THE ORIGIN OP IGNEOUS ROCKS. 
159 
has been suggested—first by Lagorio in explanation of an 
extremely local differentiation between the center and sides 
of some dikes, arid afterwards by Teall as a possible explana¬ 
tion of the differentiation of large bodies of magma. It is 
the application of the principle demonstrated by Soret, who 
found that if two parts of a solution of any salt be kept at 
different temperatures there will be a concentration of the 
salt in the cooler part of the solution. This is in accord 
with the law of osmotic pressure in dilute solutions, which 
van’t Hoff has shown is analogous to the law of gaseous 
tension.* 
In the case of certain dikes Lagorio pointed out that the 
ferromagnesian elements had been partially concentrated 
along the sides of the dikes. The same thing has been ob¬ 
served by the writer in one of the dikes in the granular core 
of the Crandall volcano. The more silicious character of the 
walls of certain dikes at Santorin has been explained b}" 
Fouque f by the supposition that the glassy sahlband was 
formed by the chilling of the first portion of the magma 
erupted through the fissure and its adhesion to the walls of 
the fissure, the central part of the dike being composed of 
a later portion of the magma. He further suggests that the 
upper part of a body of magma may be lighter than the 
lower portion, and may consist of a greater amount of sili¬ 
cious fluid magma, while the lower part carries more crystals. 
Chemical equilibrium and pressure .—It is in accord with 
the principles of thermo-dynamics as pointed out by Gouy 
and Chaperon,! to which allusion is made in Vogt’s paper, 
that there should be a change of density in a solution from 
the upper surface downward, and that the concentration 
varies proportionally to the depth. They conclude that 
* Van’t Hoff (J. H.) Die Rolle des osmotischen Druckes in der Ana- 
logie zwischen Losungen und Gasen. Zeitschr. f. Phys. Chemie. 8°. 
Leipzig, 1887, vol. 1, p. 481. 
fFouqu6 (F.) Santorin et ses Eruptions. 4°. Paris, 1879, p. 304. 
t Gouy (—.) and Chaperon (G.) Sur la concentration des dissolutions 
par la pesanteur. Ann. de Chimie et de Physique. 8°. Paris, 1887, 6th 
Ser., vol. 12, pp. 384-393. 
