272 Scientific Intelligence. 
“We may classify the explanations which have been suggested 
to account for the differentiation of lavas into two groups:—those 
which rely upon some process of selective crystallization, and 
those which premise some kind of separation taking place in a 
liquid magma prior to the commencement of crystallization and, 
consolidation. . 
‘“‘ Differentiation during crystallization has been referred to two 
distinct causes. Prof. Vogt, in order to account for the deposits 
so. rich in magnetite occurring in Scandinavia and elsewhere, has 
put forward the theory that the remarkable magnetic properties of 
the iron-spinellid may account for its segregation, and that the 
mutual attraction of the magnetite-particles may lead to their 
accumulation at certain points in the still fluid magma. But this 
ingenious suggestion omits a very important consideration. It 
has been shown by Prof. Riicker, that both metallic iron and 
magnetite, when heated, rapidly lose their magnetic properties, 
and that at a dull red heat all phenomena both of attraction and 
polarity disappear alike in metallic iron and in magnetite. Such 
being the case, it seems impossible to conceive of such a degree 
of heating of a magma as would permit of internal movements, 
and yet would not at the same time destroy the attractive proper- 
ties of the magnetite. 
‘‘The other suggested method by which differentiation may be 
brought about in a magma during the process of crystallization, 
is based on the fact that, as a general rule, the more basic miner- 
als crystallize out of a molten mass before those of acid composi- 
tion, and that, as a consequence of this, the matter left liquid con- 
tinually tends to become more and more acid in composition. That 
in these masses of molten rock consisting of basic crystals diffused 
through an acid menstruum, a process of partial or complete 
liquation may occur, is shown by the phenomena which have been 
observed at Santorin, Krakatoa, Beinn Hiant, Garabal Hill, ete. 
This method of differentiation, while it affords a satisfactory 
mode of explanation of the first class of composite dykes—that so 
well described by Lawson and Vogt—is clearly inapplicable to 
the cases in Arran, where the porphyritic crystals and glassy 
basis in the two constituent rocks are so strikingly contrasted one 
with another. 
“We are thus compelled to fall back upon the view that an 
actual separation takes place among the materials of a molten 
magma before the work of crystallization has commenced ; and I 
may, in conclusion, pass in review the several suggestions that 
have been offered to account for such action in a mixed mass of 
molten silicates. 
‘‘ Bunsen and many authors since his time have dwelt upon the 
fact that such molten masses of silicates are really solutions, and - 
must obey all the laws which govern solvent action. Guthrie and 
Lagorio have especially insisted on this view of the subject. 
‘‘Many writers on the question have been impressed by the 
view that the two magmas into which a mass of molten silicates 
may be supposed to break up must vary greatly in density; and 
