PETROGRAPHY OF THE KEWEENAWAN 643 



in some sections, chlorite in others, but the two seldom develop 

 together. 



The plagioclase is labradorite as determined by the maximum 

 extinction angles, in sections perpendicular to the albite twinning. 

 Zonal growth is not rare. The alteration of the feldspar is rather 

 different from the average case described by Pumpelly. Prehnite 

 is not a common product. Aside from a general dusty appearance, 

 a granular light-green mineral develops in the central zone as if it 

 were more easily altered than the outer zone. In extreme cases a 

 complete pseudomorph results, faintly outlined in a ground-mass of 

 no more definite character, and in the aggregate chlorite and ortho- 

 clase are indicated by analyses, if not optically. In a few cases 

 secondary orthoclase crystals were identified. In a rather different 

 alteration chlorite or some other green fibers grow up beside the 

 feldspar as radiating groups, which encroach upon the lathlike grains, 

 destroying their form and building a pseudo-amygdule. 



Phenocrysts of plagioclase are neither common nor very large 

 in the mottled rocks. When they appear, the composition is similar 

 to that of the smaller crystals. 



Augite is gray to brown, rarely showing the color due to titanium. 

 It is one of the last minerals to be affected by alteration, which usually 

 results in the borders becoming dark and dusty with chlorite. Rarely 

 a pseudomorph occurs, light green, pleochroic, and with the extinction 

 angles of hornblende. 



In the coarser, fresher sections, the minerals that have been identi- 

 fied fill the space so completely that no room is left for glassy matrix, 

 and none of the secondary products give the impression of resulting 

 from such glass, or occupying its place. In the finer-grained parts 

 of a flow — as near its base — a glass might be more probable, but even 

 here are seen only the extreme alteration products and very little 

 structure to indicate an original glassy condition. Another line of 

 reasoning runs against the probability of much glassy matrix. Augite 

 here appears to be one of the late crystallizations from the magma, 

 and this mineral is commonly classed as a eutectic.^ Hence it would 

 begin to crystallize only after all those materials had separated, which 

 made the fluid incapable of forming augite alone. It is hard to 



I Cf. A. C. Lane, Journal 0} Geology, XII, 87-88. 



