Pirsson — Phenocrysts of Intrusive Igneous Rocks. 273 



acid rocks and the pyroxenes of basic ones. The second class 

 may be termed recurrent phenocrysts, meaning that the 

 mineral composing them occurs both as a phenocryst and in 

 the gronndmass; Rosenbusch suggests the term in speaking 

 of the recurrence of mineral formation. The other, or first 

 class will be called here monogenetic phenocrysts. This dis- 

 tinction is of importance, as it is chiefly with the recurrent 

 phenocrysts that we shall have to deal. 



The proofs that all phenocrysts of intrusive igneous rocks 

 are not intratelluric, but have in many cases been formed in 

 place, and contemporaneously with other constituents may be 

 divided into two classes, those which are megascopic, that is 

 to say, geological or may be observed in the held, and those 

 which are microscopic, or to be gathered by a study of thin 

 sections. The geological proofs will be considered first. 



Evidence of contact zones. — In the case of many laccoliths, 

 dikes, sheets and other intrusive masses of igneous rocks of 

 the more acid feldspathic kinds, it has been observed that 

 while the main rock masses are highly porphyritic, often contain- 

 ing phenocrysts of great size, they pass into peripheral zones, 

 which may attain considerable width, in which such pheno- 

 crysts are entirely wanting. The border zone is often observed 

 to be dense, aphanitic and totally without phenocrysts either 

 great or small — the constituents are all of one period of forma- 

 tion and allotriomorphic in structure.* 



But if the magma contained phenocrysts before coming to 

 rest, they should occur in the border zone as well as elsewhere. 

 It will not do to suggest two separate intrusions in these cases, 

 for the geological "mise en place" and the whole character of 

 the occurrences, utterly forbids such a supposition. j^Tor will 

 it do to say that they previously existed in the border zone, 

 but were redissolved. For the border zone being the part 

 cooled first and most quickly, if they previously existed in it, 

 then there is especially the place where they should occur. If 

 any re-solution should take place, it would be in the center, not 

 at the sides. JSTor will it do to partially compromise tlje 

 matter by saying that in the original magma they were very 

 small and grew greatly after the magma came to rest. 



No doubt this often does happen, but in many cases, as 

 stated above, there are none in the contact zone, nothing to 

 represent the thickly swarming masses of them in the central 

 portion. In these cases the writer sees no escape from the 

 conclusion, that the phenocrysts were formed in place. The 

 evidence is most common and striking in regard to phenocrysts 

 of the recurrent type, but it has been also observed in those 

 of the monogenetic type. 



* Zirkel mentions similar instances, loc. cit., p. 740. 





