Pro/. Sollas — Consolidation of Minerals in Igneous Rocks. 295 



II. — The Order of Consolidation of tue Mineral Constituents 

 OF Igneous Eocks. 



By Professor W. J. Sollas, LL.D., D.Sc, F.R.S., V.P.G.S. 



rpHE order in which the various mineral constituents of an igneous 

 X rock may crystallize out from an igneous magma offers to the 

 petrologist a problem of great difficulty and complexity. It is 

 generally admitted that the order of consolidation is not wholly 

 determined by the order of the fusion-points of the constituents, 

 and with this admission the fusion-points have come to be con- 

 sistently disregarded, as though they might safely be left out of 

 account. That this is not the case has of late become strongly 

 impressed upon me, especially after a consideration of the important 

 data obtained by Mr. Ralph Cusaclc, B.A,,' who, by means of 

 Professor Joly's meldometer, has determined the precise temperature 

 of fusion of most of the rock-forming minerals. 



In the following table I have arranged the more important of the 

 minerals in question in the order of their fusion-points, as determined 

 by Mr. Cusack. 



Fusion Point. 



If for the moment we disregard quartz, there is seen to be on the 

 whole a general correspondence between the order of fusion-points 

 and the order of consolidation ; but this is by no means constant, 

 and is subject to several important exceptions. Zircon is well 

 known as a very early product of most magmas, and this accords 

 with the very high temperature of its fusion-point. Apatite and 

 sphene, however, often appear earlier than their fusion-point would 

 indicate. Olivine, with the highest fusion-point among the silicates, 

 also usually crystallizes out at an early stage, though not always 

 before leucite, of which the fusion-point is lower by 100° C. It 

 is true that petrographers are not wholly agreed as to the relation 

 in which leucite stands to olivine in this respect, but the balance 

 of evidence appears to show that both may crystallize out together, 

 or that leucite may actually precede olivine. It is of importance 

 to notice in connection with this that the leucite and olivine on 

 which Mr. Cusack experimented were both derived from Vesuvian 

 rocks ; had this not been the case the argument might have been 

 maintained that the particular species of these minerals which enter 

 into the composition of the lavas of Vesuvius may differ in chemical 

 composition from those of which the fusion-points have been deter- 

 mined, and therefore may differ also in fusion-point. Enstatite 



1 Proc. Roy. Irish Acad., ser. iii, vol. iv, p. 411 (1897). 



