1883.] PROCEEDINGS OF UNITED STATES NATIONAL MUSEUM. 193 



forms of the altered olivine abutting closely against the fresh augite, 

 while the line of separation is perfectly sharp and distinct, as indicated 

 by the continuous curved line in the sketch. Here the portions marked 

 (a) represent in each case portion of a single augite individual. More 

 careful inspection, however, shows that in nearly every instance the 

 augite is surrounded more or less completely by a narrow and very ir- 

 regular border, which projects in the form of sharp teeth or tongue- 

 like prolongations for a considerable distance iuto the .serpentine 

 (olivine) granules. This is shown in the portions marked (a) in the 

 sketches, and is very conspicuous when the section is viewed between 

 crossed nicols. This irregular border I am inclined to consider a true 

 secondary growth of augite, formed since the consolidation of the rock 

 and analogous to the hornblendic, feld spathic, and quartzose enlarge- 

 ments described by Beeke,* Irving, t and Van Hise.t I am led to these 

 conclusions from a consideration of the following facts: (1) It would 

 seem extremely improbable that the augite first separated from the 

 molten magma in such irregular forms ; (2) the original outline of the 

 augite is perfectly sharp and smooth, eminently characteristic of augite 

 outlines in this class of rocks; (3) the new portion is much the lighter 

 in color, beiug, in fact, so nearly colorless as at first to be wholly over- 

 looked when examiub'g the section by ordinary light ; (4) it projects 

 in very irregular and jagged forms into the serpentine (olivine: the 

 dotted areas in the sketch). Indeed, its appearance is such as to sug- 

 gest that not only was its formation subsequent to the consolidation of 

 the rock, but that it is an accompaniment of the alteration, the sharp, 

 tooth like edges projecting into the olivine along the curvilinear lines 

 of fracture much like the ordinary beginnings of serpen tinization. The 

 new growth in all cases possesses the same crystallographic orientation 

 as the original, the entire mass as figured extinguishing simultaneously 

 between crossed nicols. That the growth is augite, and not hornblende, 

 as in the eases described by Van Hise, is shown by its colors of polari- 

 zation, which are identical with those of the augite and of equal inten- 

 sity, and by the angles of extinction, which are the same as that of the 

 original augite. In some cases the new growth takes on beautifully 

 delicate and branched forms, the mineral ramifying along the fracture 

 lines of the olivine in such a way as to remotely resemble the eozoon 

 structure.* 



I have gone so much iuto details regarding these structures for the 

 reason that, so far as I am awar^, the phenomena of secondary enlarge- 

 ments of augite have never before been observed. Indeed, the well- 

 known habit of the mineral in passing into uralitie hornblende has, I 



- Miu. u. Pet. Mittheil., Vol. v, 1883. 



t Bull. U. S. Geol. Survey. No. 8, 1884. 



t Aiu. Jour. Sci., May, 1837. 



* The above described peculiarity of the augitic constituent was made the subject 

 of a brief paper by the writer iu the American Journal of Scieuce for Jane, 1888, p. 

 488. 



Proc. N. M. 88 13 UjU ^UULr *«? : 



