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XX. 
THEORY OF THE ORDER OF FORMATION OF SILICATES 
IN IGNEOUS ROCKS. By J. JOLY, D.Sc., F.R.S., Hon. Sec. 
R.D.S., Professor of Geology and Mineralogy, Trinity College, 
Dublin. 
[Read MArcu 21; Received for Publication Marcu 23 ; 
Published May 12, 1900.] 
Ir is well known to petrologists that the order of solidification of 
the silicates of igneous rocks is not that of their melting points; 
and the fact has been stated generally by Prof. H. Rosenbusch 
that this order is that of decreasing basicity. That is to say, the 
minerals successively formed contain ever increasing quantities of 
the acidic constituent, silica. Thus (1) ores, (2) ferro-magnesian 
minerals, (3) felspathic minerals, (4) quartz, is the order of 
solidification. Within these groups the rule is also followed: thus, 
in the second group, olivine precedes biotite, and biotite precedes 
pyroxene. In the third group, the less acidic triclinic felspars 
precede the monoclinic. 
Again, in porphyritic rocks, in which interruption due to 
physical causes in the slow process of segregation has occurred, 
the minerals of the second consolidation obey the same general 
rule. 
To the general rule there are exceptions—or apparent excep- 
tions. Thus, some of the minerals of the second and third groups 
may appear in inverted order in the dolerites and gabbros. 
Again, in pegmatitic and ophitic structures, simultaneous crystal- 
lization of quartz and felspar, or of pyroxene and felspar, may 
occur. 
When it is remembered that rock formation was often carried 
on under conditions sufficient to magnify the importance of the 
volume relations of the constituents, and under conditions of 
quiescence favouring super-cooling, the fact that other than the 
intrinsic chemical and physical properties did not always govern 
the result, is not to be wondered at. ‘The object of this note has, 
however, reference to the rule, not to the exceptions. 
