272 Pirsson — Phenocrysts of Intrusive Igneous Rocks. 



contrary in many cases they have been formed in place, and 

 are of contemporaneous origin with the other constituents of 

 the rocks, and the nature of the proofs is partly such that they 

 can be applied only to intrusive rocks, and partly they have 

 not been observed in lavas. 



The idea that phenocrysts are not necessarily intratelluric 

 has been expressed by previous writers, and Zirkel* indeed 

 gives a very full discussion of the subject, while Crossf also 

 mentions this belief in alluding to orthoclase phenocrysts which 

 are found in certain laccoliths but not in the peripheral portions. 

 But the subject does not appear to have received in general 

 the attention that its importance merits, and the writer is 

 therefore led to present his own observations bearing on this 

 point, and the deductions he has drawn from them as a contri- 

 bution to the subject. They have been gathered during the 

 progress of the geological work in Montana, by Mr. W. H. 

 Weed and the writer, and the details have mainly appeared in 

 our publications on that region, but they are now gathered 

 together and generalized for use upon this question. Similar 

 proofs of course occur elsewhere, and they are indeed of 

 general application. 



Meaning of intratelluric. — The term "intratelluric" needs 

 a moment's attention. It is probable that different writers 

 have used it with somewhat different ideas, differing as to 

 degree, not as to kind. Nowhere has the writer been able to 

 find a precise definition by an author of what his idea of intra- 

 telluric is, but it is certain from the context, that in all cases, 

 as regards time and niveau, it means an earlier period and 

 greater depth of the magma than that in which it came to 

 rest. Thus in a laccolith formed under a great thickness of 

 sediments, the phenocrysts would be spoken of as the con- 

 stituents formed in the intratelluric period, that is a lower, 

 earlier period of the magma and in contrast to the groundmass, 

 which is never spoken of in these cases as intratelluric. 



Whatever exact ideas writers may have had regarding this 

 term, it may be taken as well assumed that it means a deeper and 

 therefore earlier stage of the magma in the earth's crust, than any 

 in which the intrusive porphyritic rocks have been formed. 



Two classes of phenocrysts. — Phenocrysts may be divided 

 into two general classes. First, those which are found only 

 as distinct porphyritic crystals and do not occur as a mineral 

 component of the groundmass, like the micas and hornblendes 

 of many acid rocks, and the olivines of many basic ones ; and 

 second, those minerals which are formed both as phenocrysts 

 and also in the groundmass, like the quartz and feldspar of 



* Lehrbuch der Petrographie. vol. i, p. 737 et seq , 1893. 



f Laccolitic Mountain Groups, U. S. Geol. Surv., 14th Ann. Rep., 1895, p. 23k 



