78 PEOP. J. W. JUDD ON TEETIAET 



seated mass. But when we bear in mind that a crystal is only- 

 stable so long as it remains nnder the exact conditions which sur- 

 rounded it when it was formed (and this conclusion the researches 

 of Klein and others upon boracite, leucite, &c., give us ample 

 ground for assuming), then it may be anticipated that the corrosion 

 of a porphyritic crystal may not only be due to a change in the 

 composition of its enveloping magma, but can be equally brought 

 about if the crystal and the magma are exposed to new conditions. 



Without attempting, then, to decide the mode of origin of the por- 

 phyritic structure in each individual instance, we have sufficient 

 grounds for inferring that while, in some cases, the porphyritic 

 crystals have clearly undergone transport, and may have been 

 brought up from a more deeply-seated source, in others all the 

 phenomena point to consolidation of the materials of the rock at 

 two different periods, when the mass was subjected to dissimilar 

 conditions. 



The glomero-porphyritic structure may have originated either in 

 the one way or the other. A troctolite may have been broken up, 

 and its fragments have been included in a magma which crystallized 

 as an ophitic dolerite; or the anorthite and olivine may have separated 

 around certain centres, and new conditions have induced the crys- 

 tallization of the rest of the magma as an ophitic dolerite. 



X. Peotciples which have goveened the CetSTAIiLIZATION 

 OP the Eocks. 



Many petrographers have endeavoured to determine the exact 

 order in which the crystals have been developed in an igneous rock 

 by observations based on the enclosure of individuals of one species 

 in those of another species, or of their mutual interference during 

 growth. Professor E-osenbusch has lately insisted on the great 

 value and importance of this kind of investigation. He has called 

 especial attention to the fact that there is a remarkable contrast 

 between the rocks of basic composition and those of more acid 

 character, in the order in which their constituent minerals seem to 

 have separated from their respective magmas *. 



In the intermediate and acid rocks, the more basic minerals 

 separate out first, so that the glassy ground-mass is continually 

 acquiring a more acid composition, by the extraction of the bases 

 faster than the silica. But in the basic rocks, we find that this rule 

 does not hold good. 



The truth of this statement is admirably illustrated by the 

 rocks we are now studying. In the gabbros and ophitic dolerites it 

 is evident that the first mineral which crystallized out (after the 

 apatite) was the felspar, and not the ferro-magnesian silicates. 



But a careful study of the whole series of these British and Irish 

 rocks brings to light some very important principles, which seem to 

 govern the crystallization of the basic magmas. It is evident that 

 the separation of the different classes of minerals is not determined, 

 solely, by chemical affinities. The conditions under which consoli- 

 * Neues Jahrb. fiir Min. &c., 1882, ii. pp. 1-16. 



