34:2 C. H. Warren — Mineralogical Notes. 



parallel to the surface, although it is occasionally seen lying 

 along cleavage planes in the fayalite. On one specimen where 

 a considerable amount of the lepidomelane was developed, a 

 large number of zircon crystals, one to three millimeters in 

 diameter, were embedded. The mica corresponds to the variety 

 annite, described from this locality by Cooke/^ It fuses to a 

 black magnetic globule, gelatinizes with acids, and reacts for 

 aluminium, potash, and a little magnesium. Its axial angle 

 in oil (refractive index 1*515) was found to be 8° — 24'. 

 This mica is probably not a reaction product like the antho- 

 phyllite. It is present in other parts of the pegmatite of 

 the quarries, and its occurrence in connection with the fayalite 

 is indicative of nothing more than of the tendency of theferro- 

 magnesian and other minerals belonging to an early period 

 of crystallization to collect about anything that might have 

 induced crystallization, in this case the mass of basic iron sili- 

 cate. The presence of the zircon strengthens this view. 



Two theories may be advanced regarding the origin of the 

 Rockport fayalite ; first, that it is a basic inclusion which has 

 been thoroughly fused and recrystallized under conditions 

 which led to the development of a coarsely crystalline texture 

 like that of the pegmatite itself, during which process of crystal- 

 lization the magnetite segregated toward the margin and the 

 anthophyllite was developed : second, that the fayalite was 

 formed from the pegmatite by the action of superheated vapor 

 or steam under pressure as suggested by Iddingsf for the faya- 

 lite crystals occurring in the lithophyses of the obsidian of the 

 Yellowstone National Park. After its formation and segrega- 

 tion, the temperature and pressure diminished, allowing the 

 crystallization, etc. It is perhaps hard to understand just how 

 the segregation took place, and if it did, it would seem as if 

 smaller masses of a similar nature should exist in the peg- 

 matite. None have been found, however, except the one before 

 alluded to. 



In favor of the inclusion theory may be cited the occurrence 

 of fayalite described by Gmelin.:j; He describes the mineral 

 as an enclosed mass in trachytic lava at Fayal. It is stated that 

 the mass showed evidences of fusion and that it was filled with 

 bubbles in places. There is a possible analogy between the 

 two occurrences. 



The inclusion theory calls for the existence of a rock which 

 is nearly a pure orthosilicate of iron in composition, a supposi- 

 tion that seems rash. The second theory seems the most 

 reasonable to the author. 



*This Journal, xliii, 222, 1867. 

 t U. S. G. S., 7th Ann. Report, p. 280. 



X Chemische Untersuchung des Fayalits, Poggendorff Annalen, li, 160, 

 1840. 



