476 C. H. Warren — Alteration of Augite- Ilmenite Groups. 



closely similar rock, cumberlandite (rhodose), from the present 

 locality, a fact to which the present writer has more recently 

 again called attention. Its occurrence has also been noted else- 

 where, particularly in certain European " Flascher gabbros." 

 Its development as a secondary mineral jointly from the con- 

 stituents of ilmenite and plagioclase does not, however, appear to 

 have received much attention in standard works on Petrography, 

 although its importance as bearing on the presence of biotite 

 in metamorphic rocks is obvious. 



The effect of superficial alteration of this type is a gradual 

 change of the biotite (to some extent- the hornblende) to chlorite 

 accompanied by the formation of epidote. Kaolin, calcite, and 

 limonite also develop. 



Second type of altered gabbro. — 'This type may be character- 

 ized, in distinction to the above, as one in which the ilmenite 

 has suffered a leucoxenic alteration as well as a biotitic and 

 hornblendic one, and in which the augite, besides a passage to 

 the fibrous aggregate, shows a direct change to a more or less 

 compact green, or brown hornblende. Specimens of this type 

 have evidently suffered more severely from shearing. The 

 feldspar crystals are frequently crushed and are very generally 

 filled with saussuritic material, in addition to secondary silicates 

 more directly derived from the alteration of the augite and ore. 

 The saussuritic material consists essentially of epidote and 

 zoisite with some muscovite and hornblende. 



The change of the augite to the same fibrous aggregate as 

 previously described may be seen clearly and is unquestionably 

 a common one in this type. Another change, sometimes in the 

 same crystal, to a semi-compact pale, bluish green hornblende, 

 which assumes a deeper blue color where it is in contact with 

 the plagioclase, is of frequent occurrence. This hornblende 

 examined with high powers shows an indistinct reedy structure 

 and is filled with minute crystallites of other minerals, iron 

 oxides and epidote chiefly. Irregularly throughout its mass, 

 somewhat divergent fibrous patches may be noted which in 

 color and appearance seem to be identical with fibrous amphi- 

 bole formed directly from the augite. 



These may result directly from the alteration of the green 

 hornblende or perhaps simultaneously with it from the augite. 

 A massive reddish to yellowish brown hornblende may also be 

 seen forming directly from the augite. The brown hornblende 

 very often passes sharply in a bright blue variety and also into a 

 practically colorless mineral which seems also to be an amphibole 

 in its character although its double refraction is abnormally 

 low. The fibrous secondary hornblende formed directly from 

 the augite, the green and the brown hornblende, as well as the 

 colorless amphibole, all suffer a further change into a confused 



