C. H. Warren — Alteration of Augite-llmenite Groups. 475 



variety may well be due to the entrance into the molecule of a 

 different proportion of ferric iron and titanium. 2. The for- 

 mation of biotite, for which both ferrous and ferric iron were 

 derived from the ilmenite, alumina and alkalies from the 

 feldspar, and perhaps magnesium from the augite or fibrous 

 hornblende. The titanium, originally combined with the iron 

 used up in the formation of biotite, probably also enters into 

 this latter mineral ; at all events, leucoxene or other titanium- 

 bearing minerals are absent. 



The water given off by the rock on intense ignition reacts 

 acid, indicating the presence of fluorine. This is probably 

 present in the hornblende and biotite and it is possible that it 

 may have originally been derived from emanations from the 

 adjoining granite intrusives, which are believed to be later than 

 the gabbro and are known to contain considerable fluorite in 

 places. The effect of even an exceedingly small amount of 

 fluorine in promoting changes like those recorded here would 

 be unquestionably great and is worthy of consideration. 



The occurrence of secondary biotite and hornblende about 

 magnetite or ilmenite associated with augite in a manner which 

 appears somewhat similar to the one described here, has been 

 noted very briefly by Mr. S. Allport in an article " On the 

 Metamorphic Eock Surrounding Land's End Mass of Granite, 

 Tolcarn."* Mr. Allport describes brownish spots having the 

 mode of occurrence of magnetite and containing centers of 

 magnetite " surrounded by innumerable minute flakes of brown 

 hornblende or mica, while a short distance is usually green — a 

 fact clearly indicating the diffusion of ferric oxide." Again, 

 " Augite was abundant and has been converted into a brown 

 granular substance.-' Dr. Gr. H. Hawes, in " The Mineralogy 

 and Lithology of ^N"ew Hampshire,+ describes and figures mag- 

 netite or ilmenite grains surrounded by folise of biotite radially 

 arranged in fan-shaped aggregates with the iron oxide as a 

 nucleus, as of very common occurrence in the " syenite " near 

 Jackson, X. H. He expresses the opinion that biotite may be 

 very commonly a secondary product of this kind. Again, 

 "\TadsworthJ recognized it as a secondary mineral developed 

 from magnetite or ilmenite and the surrounding feldspar and 

 more rarely from pyroxene, in some of the gabbros described by 

 him from Minnesota. Dr. Wadsworth also notes its occurrence 

 as a secondary mineral from titanic iron ore and plagioclase in 

 the basic titaniferous rock from Taberg, Sweden, and in the 



* Quart. Jour. Geol. Soc, xxxii, p. 420, 1876. Also referred to and figured 

 bv Teall (British Petrography, plate xvii). 



f Geology of New Hampshire, C. H. Hitchcock, 1878, Part IV, p. 205, fig. 6, 

 plate xi. 



JGfcol. and Nat. Hist, of Minn., Bull. No. 2, St. Paul, pp. 65 and 90, 1878. 



