472 0. II. Warren — Alteration of Augite- Ilmenite Groups. 



Microscopic characteristics of the gabbro — least metamor- 

 phosed types. — In addition to this being the least altered of 

 any of the types examined, it may be designated as one whose 

 chief distinguishing features are a large development of 

 secondary biotite and the absence of leucoxenic alteration of 

 the ilmenite. In thin section the feldspar substance is com- 

 paratively fresh although occasional patches of sericitic and 

 sanssuritic material may be noted, and the crystals are generally 

 characterized by the presence of a brown pigment. The crys- 

 tals show abundant evidence of mechanical strains, 



The augite appears to have been the first constituent that 

 yielded to alteration, and although by far the greater portion 

 of it has gone over to secondary minerals, occasional crystals 

 may still be seen in intermediate stages of alteration. In the 

 thin sections studied, the dominant mode of alteration (if not 

 indeed the only one in this type) is to a more or less confused 

 fibrous aggregate of hornblende. Under low powers this has 

 a cloudy appearance and is of a light yellowish brown color. 

 With higher powers the fibers have a sub-parallel, also some- 

 what divergent arrangement, and possess in general a pale 

 green or yellowish green pleochroism. The change to this sub- 

 stance begins about the edges, along cleavages or fractures, 

 and encroaches in a quite irregular manner on the augite sub- 

 stance. The'amphibole often assumes a distinctly greenish or 

 bluish green color next the plagioclase. Instances may also 

 be noted where the whole aggregate has a more compact tex- 

 ture while at the same time the color is a deeper brown. 

 Small crystals of a yellowish to reddish brown, massive horn- 

 blende may also be seen, which are undoubtedly secondary 

 and strongly suggestive of a direct formation from the augite. 



The change of the augite to the fibrous form is followed by 

 a further and more important change, which often begins 

 before the original augite has entirely disappeared and in 

 which are involved reactions with constituents from the ilmen- 

 ite and plagioclase. The space originally occupied by the 

 augite as well as a part of that occupied by the ilmenite and 

 plagioclase becomes filled with an aggregate composed of 

 hornblende, biotite and particles of ore, the latter largely 

 residual although possibly to some extent secondary. Where 

 augite grains unaccompanied by ilmenite have suffered altera- 

 tion, biotite is characteristically lacking. 



This hornblende is strikingly different in appearance from 

 the fibrous form previously described, and consists essentially 

 of an aggregate of small prismatic crystals and fibers exhibit- 

 ing a distinct, though not very strong, bluish green pleochroism. 

 Associated with this is a variable amount, often considerable, 

 of hornblende in the form of prismatic crystals of relatively 



