380 REroRT—1905. 
minerals mica, felspar, and quartz, and why is the least fusible mineral the last 
to crystallise ? 
It will scarcely be possible for me to deal in this Address with more than the 
second of the two problems, but it will be apparent from the somewhat parallel 
case of the salt deposits that the mere order and manner of crystallisation of a 
mass of molten silicates must be a sufficiently complex problem to exhaust our 
attention for the present. 
Magmatic Differentiation. 
If we are to consider only recent experiments which have a bearing upon the 
problems of rock magmas, it is not necessary to say much about the first great 
petrographical problem, that of the differentiation of magmas into various rock 
types; for in this connection very few experiments have been made, and prac- 
tically none of recent date, Observations of the facts as they present themselves 
in the field accumulate every day; almost every important petrographical region 
is being studied with the particular object of determining the mutual relations of 
its rock masses and the factors which have contributed to their differentiation. 
They have been ably discussed by Becke, Brégger, Becker, Cole, Harker, Iddings, 
Judd, Lacroix, Lévy, Pirsson, Rosenbusch, Teall, Washington, Zirkel, and many 
others ; appeal has been made to the action of gravity, of temperature differences, 
of diffusion, of electric currents, of fractional crystallisation, of refusion, of chemi- 
cally combined water, of absorption of the country rock; but with the exception 
of a single case, observed in the glassworks of Targowek, in which the top of a 
molten glass was found to contain less lime and more silica than the bottom, — 
and some observations by Doelter upon boron-glass, there is scarcely a single 
experiment upon silicates which really bears directly on the question. That 
artificial glasses are far from homogeneous is known to glass-workers and to 
makers of lenses, but there is nothing comparable with the splitting of a magma 
into two or three distinct liquids which solidify as different rocks. 
It is in the case of laccolites that the problem ought to present itself in the 
simplest form, for we may regard them as basins of igneous rock which have been 
practically imprisoned within solid walls and have crystallised i 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 central 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 
ithe former; and those theories which assume that different liquids have separated 
ifrom 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 throughout 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 metals, metallic oxides, and sulphides have been produced by mag- 
matic differentiation from deep-seated magmas which now constitute basic rocks 
‘associated with them. But this does not explain how the mass which has crystal- 
ised 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 con- 
‘vection 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 
