150 Scientific Proceedings, Royal Dublin Society. 



ing the ultimate mineralogical composition of the solidifying rock. 

 Let us take two groups of the mineral elements of Yesuvian 

 essential ejectamenta ; we have leucite antagonistic to amphibole, 

 nepheline, and mica, all competing for the potash. Now, in the 

 pumices of the great explosive eruptions of Phases III. and VI. we 

 find amphibole, sanidine, and biotite using up the potash, and 

 being the principal crystalline ingredients, whereas in the lavas 

 that cooled under quite different conditions we find these minerals 

 reduced to a minimum, whilst all the potash has been seized upon 

 by the leucite, and sometimes a little nepheline. How can we 

 account for such phenomena, otherwise than in change of condi- 

 tions ? Again, we find pyroxene, antagonistic to olivine, amphi- 

 bole, and biotite, competing for the magnesia. Again, in the 

 Yesuvian pumices, amphibole and mica prevail, as these had pro- 

 bably formed under great pressure, whilst in the same pumices that 

 escaped more slowly, and in the lavas, it is the pyroxene that mono- 

 polized the magnesia. We know that olivine (?), amphibole, and 

 biotite are met with in their greatest perfection in plutonic rocks, 

 whilst pyroxene is remarkably characteristic of rocks slowly cooled 

 near the surface, and under low pressure. The fact of the former 

 of these having resisted all attempts at artificial production points 

 to conditions which have not yet been adopted in the laboratory, 

 whilst leucite and augite are produced with ease and certainty. 

 We therefore must conclude that antagonism of mineral species in 

 crystallizing from a medium depends not only on the composition 

 of that medium, but also of the surrounding physical conditions. 

 Prof. Haughton 1 admits that, according to his theory, olivine ought 

 to prevail, as it has only to contest for iron and magnesia, whilst 

 pyroxene, amphibole, and biotite, are weakened in the additional 

 fight for lime or alumina. He attempts to explain this by a 

 theoretical principle which he calls that of minimum paste, which 

 would not have been requisite had the physical conditions been 

 taken into account. Again, this theory in its incomplete form is 

 proved insufficient by the joint author, Prof. E. Hull, 2 in the same 

 memoir, although it was undoubtedly a great step in the direction 

 of an important principle. 



1 Op. cit. 9 Op. cit., p. 141. 



