426 Reports and Proceedings — British Association. 



The second theoiy, that of liquid diflferentiation, 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 con- 

 sisting 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. 



Many 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 explaining 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 proportions 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 6*i°, 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 magma as 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 shoukinite. 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 tho 

 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 com- 

 promise 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 crystallises 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. 



