C. H. Warren — Alteration of Augite-Ilmenite Groups. 471 



now somewhat obscured by superficial alteration, but it seems 

 clear that their intensity varied quite irregularly, being stronger 

 toward the northern and northwestern contacts, and in inde- 

 finite zones through the body of the mass. 



Specimens showing the least metamorphism and but little 

 superficial alteration may be found in the north-central part 

 of the area along an old car track, where the surface of some 

 of the ledges has been blasted away. The rock here is of a dark, 

 greenish brown color, and breaks with a more or less distinct 

 cleavage, owing to a rudely parallel orientation of the tabular 

 plagioclase crystals. 



The plagioclase is dark brown in color and is beautifully 

 > striated. Between the crystals are irregular patches of dark 

 green or brownish green, finely crystalline, secondary silicates 

 in which are very generally embedded lustrous grains of ilmen- 

 ite. The green material can be seen to penetrate to some 

 extent the feldspar substance. Polished surfaces, looked at 

 with a good hand lens, serve to show the texture admirably. 

 Outcrops in the southeastern part of the area show about the 

 same degree of metamorphism, although superficial alteration 

 has gone further. On exposed surfaces the feldspar becomes 

 chalky and retreats, leaving the black ore grains and their 

 matrix of secondary silicates, now of a dull, pale green color, 

 standing out in relief. 



More severely metamorphosed phases may be recognized by 

 the fact that a portion of the feldspar has changed to a dull 

 white saussurite, a change that becomes complete in the more 

 extreme types. The latter are also characterized by the 

 diminished number and size of the ore grains and by the 

 general loss of the original texture. Types representing these 

 stages may be collected a little east of the exposures of the 

 least altered type above alluded to, and from the ledges on the 

 north and south of the railroad track. Toward the northern 

 border of the area and along portions of the high ridge that 

 forms the eastern outcrop of the gabbro toward the Iron Mine 

 Hill, the rock has lost, so far as its macroscopic appearance is 

 concerned, almost every vestige of its original texture. It 

 shows an indistinct schistose structure, and has a ' mottled, 

 greenish white appearance. Chlorite, a little sericite and an 

 occasional remnant of ilmenite and feldspar are the only 

 minerals that can be distinctly identified, although the dull 

 white dense groundmass, especially in its weathering, is sug- 

 gestive of a feldspathic composition. At the contact with the 

 granite and schists on the north, the gabbro has been sheared 

 into a fissile green schist. At the extreme northeast extension 

 of the outcrops the rock has become a greenish white, fissile 

 schist. 



