410 Scientific Proceedings, Royal Dublin Society. 
consolidate according to the plutonic or Rosenbusch’s order of crys- 
tallization. If, on the other hand, the pressure is relieved (e.g. by 
rising nearer to the surface), and then, owing to expansion or other- 
wise, the temperature falls, we have the surface, or melting-point 
order of crystallization, with quartz coming out first. And if 
the pressure be suddenly or rapidly relieved by the magma rising 
in the neck of a volcano, while the temperature also falls, we 
should expect to have the minerals of high melting-point rapidly 
crystallizing out as large porphyritic crystals. Mr. J. J. H. Teall 
says': “ The first fact that strikes one is that the granular texture 
is especially characteristic of plutonic (¢.e. deep-seated) ; the por- 
phyritic texture of volcanic (?.e. surface) masses.” Also the 
numerous inclusions and many other imperfections of such porphy- 
ritic crystals may be partly accounted for by rapidity of growth. 
Mr. C. E. Stromeyer in his Paper has given several examples, 
illustrating the way in which thistheory explains cases of exceptional 
order of crystallization. That relating to the so-called “pegmatite” 
veins should appeal to members of the Royal Dublin Society who 
are familiar with the veins in the granite at Rochestown and 
Stillorgan, where idiomorphic prisms of quartz may be seen 
imbedded in crystals of felspar. I may add the example of the 
graphic structure, owing to simultaneous crystallization of quartz 
and felspar, exhibited in certain granites. Now it is certainly 
often the case that this graphic structure is associated with drusy 
cavities, apparently due to diminished pressure, near the tops of 
granite domes (e.g. in the granite of the Mournes and at St. 
David’s). We have, in fact, got down to the “eutectic pressure ”’ 
of quartz and felspar (OB on the above diagram). At greater 
depths in the same granite boss, we have an increased pressure 
(greater than OB), and, therefore, the felspar has crystallized 
before the quartz, as is usual with deep-seated plutonic rocks. If 
a molten magma comes so near the surface as to be only covered 
by a thin layer of rock, we may have so little pressure on its 
upper portions that the quartz will crystallize first and give rise 
to a quartz porphyry. If the fluid mass was of sufficient dimen- 
sions, we should be able to trace the different types of crystalli- 
zation down through the graphic into the true granitic type. 
Professor Grenville Cole tells me that the Slieve Gallion mass 
1J.J.H. Teall, “ British Petrography,’’ p. 55. London, 1886-1888. 
es 
