32 ANNALS NEW YORE ACADEMY OF SCIENCES 



above, and shows particular relationship with the biotite augite variety. 

 Williams records it from only one place (along the road to Montrose 

 Point), which is so small an area that it cannot be mapped. 



In the hand specimen, the biotite is, of course, distinguishable, but 

 these rocks cannot be told from those which carry also augite. The 

 feldspar often imparts a dark pink color to the rock, but it is as often a 

 dark gray. The grain is medium, being never very fine or very coarse. 

 A faint gneissoid structure is occasionally apparent, but the rock is gen- 

 erally quite massive. 



Under the microscope, the feldspar resembles that described under 

 norite proper. The reddish dust is usually, but not always, visible. The 

 andesine carries it most frequently; when the plagioclase becomes labra- 

 dorite, the inclusions are more grayish. The lamella? are sometimes bent 

 or broken by strain, and secondary twinning is also sometimes induced, 

 but such a degree of metamorphism is rare. The orthoclase may be al- 

 most entirely wanting, or it may again make up one third of the feldspar. 

 It sometimes becomes quite gray from inclusions. 



The biotite is present in typical habit ; it seems to be a somewhat less 

 ferriferous variety than is characteristic of the diorites, and the axial 

 angle is larger. Its pleochroism is X golden yellow, Y dark brown, Z 

 brownish or greenish black. Earely the colors are lighter than this. The 

 biotite is an especially delicate indicator of the amount of strain which 

 the rock has undergone, but, as stated above, any marked amount is rare. 



The hypersthene is, of course, the essential constituent of the whole 

 norite group. It usually occurs in stout, rounded prisms, which are idio- 

 morphic unless the ferromagnesian minerals are unduly crowded. In the 

 finest grained rocks, the crystals are small and often clustered together 

 and even in the coarser varieties, the hypersthene is sometimes present in 

 large aggregates of small irregular grains. Sometimes there is a ten- 

 dency for the ends to fray out, as it were, and alteration then begins at 

 this point. The depth of color and pleochroism vary directly with the 

 amount of iron. Enstatite rarely occurs in the norites; bronzite, with a 

 faint pleochroism, is more common; but hypersthene, of varying degrees 

 of pleochroism, color and relief, is the typical orthorhombic pyroxene. 

 X varies from deep red to reddish yellow or pink; Y from yellowish 

 brown to dirty yellow or pale yellow, and Z from bright to pale green. 

 Extinction is, of course, parallel. In basal section, it shows the charac- 

 teristic crossed prismatic cleavages parallel to (110) ; the pleochroism is 

 then rather faint, but the figure of the obtuse bisectrix distinguishes the 

 mineral from monoclinic pyroxene. The fine parting of hypersthene 

 parallel to (010) is usually well developed, and the characteristic ilmenite 



