280 Pirsson — Phenocrysts of Intrusive Igneous Rooks. 



increasing ratio. This means. shorter and shorter crystalliza- 

 tion intervals for the various components as their periods com- 

 mence. Thus the earlier ferro-magnesian components may 

 have time to grow to considerable size while the increasing 

 viscosity forces the feldspars and later components to be of 

 very small size. Thus we may have a porphyritic structure 

 with phenocrysts of the monogenetic type as a result of a 

 single process of crystallization. It may be considered a sort 

 of arrested or retarded granular structure. 



But phenocrysts of the recurrent type evidently demand a 

 further explanation and this is to be sought in two things. 

 First the writer believes that the influence of mass action is of 

 great importance and that compounds which are present in 

 relatively large or predominant amount tend to set up centers 

 of crystallization before what might otherwise be the proper 

 period for that component. The rest of the process might go 

 on as noted above and a porphyritic structure would be formed 

 with phenocrysts of the recurrent type. Phenocrysts of the 

 recurrent type would then consist of those minerals which are 

 present in largest amount, and this is generally true. Such phen- 

 ocrysts might form at any time and the earliest would be crys- 

 tallizing during the whole period and might include the other 

 constituents or exclude and orient them as previously noted. 



Second, the writer is inclined to believe that too great regu- 

 larity in the period of commencing crystallization of the dif- 

 ferent components has been ascribed to crystallizing magmas. 

 That the different minerals tend to have their proper periods 

 is undoubtedly so, but they do not always have them, and, for 

 example, centers of feldspar crystallization may be set up at 

 different times resulting in porphyritic structure as mentioned 

 above. If we dissolve salt in water and allow the solution to 

 crystallize in different beakers, we may obtain under apparently 

 similar conditions a few large crystals, many small ones or a 

 mixture of large and small. The conditions are of course not 

 exactly similar but the differences may be very slight. 



The more coarsely granular the rocks are, the more stable, 

 and even the conditions under which they have crystallized 

 and the less likelihood there is for the formation of pheno- 

 crysts — yet these do occasionally occur, as noted above in the 

 porphyritic granites. In the tine-grained rocks these conditions 

 are changed, the causes mentioned above become operative and 

 a porphyritic structure is liable to result, as it commonly does. 



Thus it seems most reasonable to regard the majority of 

 phenocrysts as formed in place though in some cases they have 

 evidently formed previously while the magma was ascending. 

 Even in these cases they have probably been formed not long 

 before. 



Yale University, New Haven, Conn, December, 1898. 



