514 GEOPHYSICS 



The physics of magmatic solutions is a great subject which is ex- 

 perimentally almost untouched, although a vast amount of geological 

 speculation has been based upon assumed properties of magmas. It 

 is only within a few months that even satisfactory melting-point 

 determinations of those most important rock-forming minerals, the 

 lime-soda feldspars, have been made. The feldspars are only one 

 series of isomorphous mineral mixtures. Their study is fundamental 

 and must be followed by that of the remaining class, i. e., the eutectics. 

 These, in my opinion, will lead to a rational classification of igneous 

 rocks, themselves mixtures and incapable of logical description except 

 in terms of standard mixtures, the eutectics. 



It appears to me highly probable, for many reasons, that the mag- 

 mas of the granular rocks are not liquids but stiff emulsions, compar- 

 able with modeling-clay, the solid constituents (perhaps free oxides) 

 being merely moistened with magmatic liquids. Such masses behave 

 mechanically like soft solids; they display some rigidity and in them 

 diffusion is reduced to a vanishing quantity. They may be ruptured 

 and the (aplitic or pegmatitic) liquid portion may then seep into the 

 cracks. Such a magma might be forced into minute fissures, as is the 

 case when clay is moulded to terra-cotta articles, and yet it would 

 support permanently, on its upper surface, rocks of superior density. 

 Only in such a magma can I comprehend the simultaneous growth of 

 crystals of various minerals; for in a liquid not exactly eutectic, the 

 formation of crystals must follow a definite order. Again, if banded 

 gneisses and gabbros had been fluid, the bands would show evidences 

 of diffusion which as a rule are absent or barely traceable in these 

 rocks. 



The relations between consanguineous massive rocks have occupied 

 a large part of the attention of geologists for many years. At one 

 time it was supposed that homogeneous liquid magmas might split up 

 into two or more homogeneous magmas by processes of molecular flow 

 due to differences of osmotic pressure. This process was called the 

 differentiation of magmas. It has been shown, however, that these 

 processes are so much slower even than heat-diffusion, that they can- 

 not be efficient beyond distances of a few centimeters. For this reason 

 Mr. Teall, 1 who first suggested the application of the Soret process 

 to account for differentiation, Professor Brogger, 2 and others, have 

 abandoned the hypothesis of differentiation on a considerable scale 

 by molecular flow. Nevertheless, observations on laccolites and other 

 occurrences leave no doubt that a single magma may solidify to differ- 

 ent though consanguineous rocks. If the separation is not molecular, 

 it is self-evident that it must be molar. The only molar currents 

 readily conceivable in a body of magma are convection currents, and 



1 Proceedings, Geological Society, London, vol. 57, 1901, p. Ixxxv. 



2 Eruptivgestiene der Kristianiagebietes, part in, p. 339 



