G. H. Warren — Alteration of Aitgite-Ilmenite Groups. 477 



aggregate of hornblende prisms exactly like that noted in the 

 previous type. Biotite has also formed in the same manner 

 but both the hornblende and the biotite, particularly the latter, 

 have suffered a more general distribution through the rock and 

 are less closely confined to the place of original formation. 

 Much of the biotite and hornblende has now suffered further 

 alteration to chlorite. 



Leucoxene is abundantly developed about the ilmenite and is 

 plainly of later origin than the biotite and hornblende. It is 

 distinctly crystalline and has the characteristics of titanite. 

 This leucoxenic alteration has brought out with great clearness 

 the reticulate structure of the ilmenite. 



In comparing what have here been called the two types of 

 alteration, the writer is led to conclude that the former is one 

 brought about under conditions of deep-seated, hydrothermal 

 action but unaccompanied by extreme shearing and crushing. 

 In the second type generally similar changes obtained for a 

 time but were succeeded by others induced by more severe 

 local dynamic action.- The latter changes are : The forma- 

 tion of leucoxene ; a greater tendency for the augite to pass 

 directly into a compact or semi-compact hornblende ; increased 

 saussuritization of the feldspar ; and a more general distribution 

 of the secondary hornblende and mica throughout the rock. 



More highly metamorphosed types. — Further metamorphism 

 and alteration of the gabbro presents little that is novel, and a 

 very general statement will suffice. The microscope reveals 

 increased crushing and saussuritization of the feldspar, a com- 

 plete change of the hornblende and biotite to chlorite and 

 epidote, an almost complete alteration of the remaining ilmenite 

 to leucoxene, and the development of some sericite. Some 

 secondary quartz and recrystallized feldspar may also be noted. 

 A more or less schistose arrangement of the constituents 

 becomes evident and the outlines of the original structures are 

 less and less distinct, until in extreme phases, from near*the 

 northern contacts, the slides show little more than a schistose 

 mass of finely crushed feldspar mingled with secondary products. 



Laboratory of Mineralogy and Petrology, 



Massachusetts Institution of Technology, Boston, Mass. 



