Reports and Proceedings — British Association. 425 



igneous rock wliich have been practically imprisoned within solid 

 walls and have crystallised in situ. There can, I think, be no doubt 

 that differentiation has generally taken place even in such basins, 

 that the margins have often a different mineralogical and chemical 

 constitution from the more centi'al portion, and that the differences 

 are greater than can be accounted for by solution of the enclosing 

 rock, and are often of a chemical nature which cannot be so 

 • explained. 



The various theories that have been propounded fall into two 

 distinct classes — those which seek the cause in the separation of 

 solid material from the liquid, so that when the latter subsequently 

 crystallised it constituted a different rock from the former; and 

 those theories which assume that different liquids have separated 

 from each other and then solidified as different rocks. 



The first conception satisfactorily explains the manner in which 

 the least soluble minerals are concentrated at the bottom or margin 

 of an igneous mass, for they naturally crystallise first where the 

 mass is coolest, or where contact with other crystals may have 

 occurred ; or even if they have been precipitated as a cloud through- 

 out the magma they must be carried about by convection currents 

 and ultimately sink together unless the magma be very viscous. 

 Most geologists will probably agree with the conclusions of Vogt 

 that some of the most important deposits of metal, metallic oxides, 

 and sulphides have been produced by magmatic differentiation from 

 deep-seated magmas which now constitute basic rocks associated with 

 them. But this does not explain how the mass which has crystallised 

 out may be not a mineral but a rock. 



The actual observations on crystallising solutions do not amount 

 to much ; it is quite clear from laboratory experiments that crystals 

 do grow by means of convection currents, which produce a flow of 

 stronger solution towards the crystal and of weaker and warmer 

 solution upwards and away from the crystal. The concentration 

 currents can easily be seen in any ordinary aqueous solution as 

 streaks in the liquid. Again, that there might be a slight difference 

 'in the concentration of the upper and lower, or of the warmer and 

 cooler parts of a solution has also been shown. That a very con- 

 siderable difference in concentration can be produced by centrifugal 

 action was proved only last year by the experiments of Calcar 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 re- 

 dissolve when the pressure is relieved, and so give rise to an under- 

 lying magma of different chemical constitution. 



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. 



