Geology and Mineralogy. 273 
several of these writers have suggested, in more or less distinct 
terms, that this difference in specific gravity may be the efficient 
cause of their separation. But until recently it was difficult to 
realize the physical principles that would cause density-differences 
to come into play as factors of differentiation. The researches of 
Gouy and Chaperon, however, supply a possible explanation of 
the phenomenon. These authors have pointed out that, in accord- 
ance with well-recognized thermodynamical principles, there 
should be—in a very great volume of a solution, especially if near 
saturation—a tendency for concentration to take place in the 
iower parts of the mass. Such vast masses of molten silicates as 
must exist in the deeper parts of the earth’s crust are precisely 
the kind of solutions in which we may expect to find the action of 
this law illustrated. 
“There is still another physical principle which has been ap- 
pealed to as affording an explanation of the differentiation which 
takes place in a liquefied mass of silicates. In the same year that 
Gouy and Chaperon demonstrated the causes that would lead to 
“the concentration of solutions by gravity,” van’t Hoff published 
his paper on ‘The Role of Osmotic Pressure in the Analogy be- 
tween Solutions and Gases.’ He argued that solutions must fol- 
low the law of gaseous tension, and that if two parts of a solution 
be maintained at different temperatures, concentration must take 
place in the cooler part. It is interesting to note that, six years 
before van’t Hoff published his general conclusions, Soret had con- 
ducted a series of experimental researches which showed that, 
when two portions of a solution are maintained at different tem- 
peratures, there is always a tendency to concentration in the colder 
part. 
‘““If, as we may assume is generally the case, the lowest portions 
of the great liquid reservoirs within the earth’s crust are at the 
highest temperature, then the principles established by Gouy and 
Chaperon and by van’t Hoff respectively would operate in con- 
trary directions and tend to neutralize each other. It is sufficient 
for our present purpose, however, to call attention to the fact that 
there are now well-recognized physical principles, in accordance 
with which differentiation must necessarily be set up in the heated 
solutions constituted by molten masses of mixed silicates, anterior 
to, and independently of, the liquation that may follow selective 
crystallization. That differentiation does take place before, as 
well as during, crystallization, the phenomena exhibited by the 
two classes of composite dykes afford interesting and striking 
proofs.” 
6. A plumbiferous Tetrahedrite ; by G. Cur. HorrmMann (com- 
municated).—The mineral here described was obtained from the 
Antelope claim, Kaslo-Slocan mining camp, West Kootanie dis- 
trict, British Columbia, Of the specimens examined—some con- 
sisted of the tetrahedrite, in question, associated with galena and 
small quantities of sphalerite and pyrite; others, of the perfectly 
pure mineral in a gangue of white opaque quartz. The latter 
furnished the material employed for the analysis. 
