TRANSACTIONS OF SECTION C. 381 
streaks in the liquid. Again, that there might be a slight difference in the con- 
centration of the upper and lower, or of the warmer and cooler parts of a solution 
has also been shown. That a very considerable difference in concentration can be 
produced by centrifugal action was proved only last year by the experiments of 
Calear and de Bruyn, in which solutions contained in rapidly rotating vessels 
became more concentrated in the portions furthest from the axis of rotation. 
Schweig has recently suggested that the crystals which fall to the bottom of a 
rock-magma may be unstable compounds, which redissolve when the pressure is 
reliéved, and so give rise to an underlying magma of different chemical consti- 
tution. 
Harker, also, some time ago, suggested the existence of horizontal layers of 
different liquid magmas above each other, thus attempting to explain the presence 
of quartz in basic rocks as due to the crystals which had sunk into the basic 
magma from a more acid magma floating upon it. 
Tke second theory, that of liquid differentiation, regards such layers as actually 
produced by the spontaneous division of a magma into two liquids of different 
composition, and if it be tenable seems more capable of explaining the geological 
facts. 
The experiments bearing on the subject are well known, and have been quoted 
by Backstrom and Teall; mixtures of phenol and water, or of aniline and water, 
which form a homogeneous solution above a certain temperature, may below that 
temperature (which is a sort of critical point of the solution) divide into two 
solutions, one consisting of phenol in excess of water, the other of water in excess 
of phenol ; and these two solutions are not miscible, but separate into two distinct 
layers. 
Piiairy pairs of substances have now been found to exhibit this incomplete 
miscibility, which varies with the temperature and may at certain temperatures 
become complete ; among them are some of the metals such as zinc, lead, bismuth, 
and silver. 
If rock-magmas can really behave in this way, there is no difficulty in explain- 
ing their differentiation ; but experiments upon fused silicates have not disclosed 
anything of the sort, though they are made far below the critical temperature. 
The case of nicotine and water, which has recently been described by Hudson, 
is remarkable and suggestive : above a temperature of 205° a mixture in equal pro- 
portions is a clear liquid; at 205° it divides into a saturated solution of nicotine 
in water floating on a saturated solution of water in nicotine; at 90° these two 
layers change places; at 64° they mix again and the liquid becomes once more 
homogeneous. 
It is, of course, possible that fused silicates at experimental temperatures 
correspond to nicotine and water below 64°, and that rock-magmas correspond to 
the same mixture at higher temperatures. 
In discussing the reasons why in laccolites of the Square Butte type the 
margin should be more basic, and in laccolites of the Magnet Cove type more acid 
than the centre, Washington regards the magmaas a mutual solution of an alumo- 
alkaline substance with a ferro-magnesian substance; whichever of these is in 
excess may be regarded as solvent, and crystallises first, for example, either the 
syenite or the shonkinite. In a laccolite where no differentiation has taken place, 
as in the Henry Mountains type, he supposes the mixture to be eutectic or such 
that they crystallise together. Pirsson, in a paper recently published upon the 
‘Highwood Mountain Laccolites of Montana,’ while attributing a greater part 
in the process to the action of convection currents, also regards the ferro-magnesian 
minerals, taken together, as constituting the solvent and crystallising first as 
shonkinite. 
In fact, stated quite baldly, these latest views tend to a compromise between 
the two theories which I have just mentioned. They regard the splitting of the 
magma as produced by a fractional crystallisation, only now the mass which 
erystallises is not a mineral but a rock; in other words, they assume that rocks 
may be dissolved in each other, and may crystallise from each other as though 
they were minerals, 
