Pirsson — Phenocrysts of Intrusive Igneous Pocks. 271 



Akt. XXYIII. — On the Phenocrysts of Intrusive Igneous 

 Pocks;- by L. Y. Piksson. 



The igneous rocks, excluding those which have solidified as 

 uncrystallized glasses, may be roughly divided into two classes, 

 the granular, composed of mineral particles of approximately 

 similar size, and the porphyritic, consisting of minerals exhibit- 

 ing a more or less perfect outward crystal form which lie em- 

 bedded in a so-called groundmass made up of much smaller, 

 usually anhedral,f particles or even wholly or in part of glass. 

 A rock belonging to this latter class is a porphyry and the 

 larger embedded crystals are called " porphyritic crystals " or 

 more conveniently "phenocrysts" a term proposed by Iddings 

 which has obtained general recognition among geologists. 



It is with respect to the origin of these phenocrysts in a certain 

 class of igneous rocks that it is proposed to treat in this article. 



Without going into details, it will be sufficient to recall that 

 many petrographers have expressed the view that the pheno- 

 crysts of porphyritic rocks, by the very fact of their size, fre- 

 quent perfection of crystal form and contrast of character to 

 that of the groundmass in which they lie embedded, are older 

 than this groundmass, have been formed under different physi- 

 cal and therefore different geological conditions, in short, not 

 in the place where they now are, but at great depths, and hence 

 they are frequently spoken of as " intratelluric" in origin.^ 



The writer has been unable to find in the literature, that 

 any sharp distinction has been drawn between the origin and 

 character of the phenocrysts of intrusive igneous rocks and 

 those of the extrusive ones or lava flows. This has probably 

 arisen from the belief that phenocrysts are always formed, as 

 stated above, at much greater depths than the place in which 

 they are now found, and that therefore their origin is similar 

 in both cases. It would thus make no difference in regard to 

 their origin and character, whether the ascending magma 

 which contained them was thrust in intrusive masses into the 

 upper portion of the earth's crust or poured out on the surface 

 in flows of lava. 



In the present article, however, it is to be carefully noted 

 that the writer confines himself to the phenocrysts of the in- 

 trusive rocks, as it is intended to show that not all phenocrysts 

 are intratelluric in the sense that they have been formed at 

 much greater depths than they now occur in, but that on the 



* Eead before the Geological Society of America, December, 1898. 



\ i. e., having crystal structure but not crystal form. 



\. Roseobusch, Mass. GTest., 3d ed., 1896, p. 553; Michel-Levy, Struct, et Class, 

 des Roches, 1889, p. 1 et seq. See also discussion by Zirkel, who does not follow 

 this view, Lehrb. der Petrogr., 2d ed., 1893, vol. i, p. 73*7 et seq. 



