54 



When the members of the second group are compared with those of the 

 third, the law of ROSENBUSCH becomes more difficult of application. The 

 following facts may be mentioned as exceptions to the law. 

 Magnetite is not always the first product of consolidation. It may be formed 

 at almost any stage in the process of consolidation. (1) Quartz is anterior in 

 date to the felspar in the case of many granites. (2) Oligoclase is sometimes 

 found as a zone surrounding orthoclase. (S) 



There can be no doubt that the conditions which govern the formation of 

 crystals in an igneous magma are extremely complex. The chemical composition 

 of the magma and the physical conditions (e.g. temperature and pressure) 

 under which consolidation is effected are probably the most important factors. 

 One of the problems of the future will be to determine how these factors 

 stand related to the resulting mineral compounds. 



We have now to consider the second point raised in the paper by 

 Professor ROSENBUSCH ; namely the recurrence of phase in typical porphy- 

 ritic rocks. In these rocks members of the second, third and fourth groups 

 occur as porphyritic elements and as constituents of the groundmass. In 

 certain quartz-porphyries we find quartz and felspar (orthoclase) occurring 

 as porphyritic elements and also as constituents of the groundmass. In 

 andesites, porphyrites and certain dolerites (e.g. those of Etna), felspar 

 (triclinic) and ferro-magnesian minerals occur under the same circumstances. 

 It is obvious that in these rocks the members of the second and third groups 

 have not been all produced at one and the same period, or under the same 

 conditions. This fact, of course, has long been recognised, and its importance 

 has been especially insisted upon by Messrs. FOUQUE & LEVY who separate 

 the original minerals into two groups : (1) those formed prior to the final 

 act of consolidation, ( 2) those formed in connection with final consolidation. 



The third point in Professor ROSENBUSCH'S paper is the proposal to 

 make the recurrence of phase the essential feature indicated by the term 

 porphyritic. M. LEVY (4) has clearly shown that in many typical granites, 

 which do not contain any definite porphyritic crystals, it is possible to 

 recognise, by the use of the microscope, that the constituents belong to two 

 distinct periods. The minerals of the earlier period are not distinguished by 

 reason of their size ; but by the fact that they show more perfect form, or else 

 occur in fragments. Again, in the basic rocks of granular aspect we find the 

 same fact illustrated. The Whin Sill, the Hett dyke and the High Green 

 dyke, for example, all of which occur in the north of England, are composed 

 of rocks which are thoroughly granular to all appearance, and yet if a large 

 series of slides be examined it will be seen that they contain here and there 

 crystals of felspar, which must have been formed before the consolidation of 

 the main mass of the rock, and which would therefore make the rock 

 porphyritic if the definition of Professor ROSENBUSCH were adopted. It is 



(1) FOUQUE & LEVY. Synthese des Mineraux et des Roches, p. 51. 



(2) J. ROTH. Allgemeine und Chemische Geologic. Band II, p. 50. 



(3) DE LA VALEE POUSSIN ET RENARD. Memoire sur les Roches dites Plutoniennes, &c., p. 175. 



(4) De quelques characteres microscopiques des roches anciennes. B.S.G.F., Se. III., Vol. III., 

 p. 199. 



