276 Pirsson — Phenocrysts of Intrusive Igneous Rocks. 



cerned, as a difference not alone of degree but of Jcind. If so, 

 what is this precise difference and why should granular rocks 

 ever possess a porphyritic structure ? 



What the significance of these crystals really is according to 

 the author's belief will be shown later on. 



Microscopical evidence. — We now come to those proofs 

 which are to be seen in the minute internal structures of the 

 phenocrysts when seen in thin section under the microscope. 

 They may be divided into two classes, external, or to be seen in 

 the minerals surrounding the phenocryst, and internal, to be 

 seen in the minerals which the phenocryst encloses. We will 

 consider the latter first. 



Internal evidence. — While the majority of phenocrysts are 

 homogeneous and free from inclusions, in a large number of 

 cases they are not, but contain the other rock minerals. These 

 are usually those of an older period of formation, such as apa- 

 tites and ores, and their explanation appears simple, but in 

 some instances the phenocrysts may enclose all the other rock 

 minerals including crystals of the same mineral as they them- 

 selves are composed of. Thus large orthoclases are seen enclos- 

 ing other orthoclases and quartz. If the enclosed mineral has 

 the same composition and a similar Orientation as the pheno- 

 cryst, it becomes of course an integral portion of the crystal 

 and the enclosure cannot be detected, but it often does not and 

 is then readily identified. In these cases it is impossible to 

 avoid the conclusion that not only was the phenocryst formed 

 in place but contemporaneously with the other minerals. At 

 such times the included minerals are scattered throughout the 

 whole extent of the phenocrysts, but often they are found only 

 in outer zones, indicating that the inner portion had formed 

 first and that later the phenocryst was growing and expanding 

 while the groundmass was crystallizing and was thus including 

 it.* The ability of crystallizing material to orient itself over 

 considerable areas and enclose the other constituents is well 

 illustrated in the poikilitic structure as seen in the quartzes of 

 rhyolites, the orthoclases of shonkinites and the hornblendes 

 and micas of the very basic rocks. 



External evidence. — But growing and expanding pheno- 

 crysts do not always enclose the other minerals of the ground- 

 mass with which they come in contact. On the contrary, like 

 all crystallizing substances, they endeavor to exclude impurities 

 and more commonly they reject and push them along as they 

 grow. Usually the minerals which make up the groundmasses 

 of porphyritic rocks have an isodiametric development, they 

 have therefore no orientation as regards form, and it is hence 

 difficult to tell whether such particles lying alongside pheno- 



* These Montana instances give additional force to those mentioned by Zirke! 

 in his discussion of the subject, loc. cit., p.. 747. 



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