GABBRO AND NORITE INTRUSIVES, 237 



This brown lioniblende, on alteration, breaks np into aggregates of 

 epidote-zoisite and light-green chlorite. 



The second kind of hornblende is the perfectly fresh, compact, com- 

 mon dark-green kind, with pleochroism varying from yellow for a, to 

 yellowish-green for 1), and to bluish-green for C; C>ti>>a. This is found 

 in very few cases. It appears in every instance to be a primary constituent. 



The third kind of hornblende may be primary, although the evidence 

 obtainable points to its secondary nature. It has a light-green color, and 

 when examined for pleocliroisra- exhibits a scarcely noticeable change. 

 This .hornblende diifers very much from the other two hornblendes 

 described, in that it is not compact, but occurs in aggregates of coarse reed- 

 like (schilfaehnliche) individuals. Such aggregates do not at all resemble 

 uralite. The individuals are far too coarse and wedge out at short distances 

 within the aggregates. The aggregates occupy irregularly shaped areas. 

 The aggregates consequently have a coarse patchy polarization. They are 

 frequently surrounded by ragged pieces of biotite, just as ai-e the plates of 

 compact hornblende. Moreover, they occur in rocks which show pressure 

 effects, and are best developed in those in which such effects are most 

 marked. The aggregates rather frequently occur with irregular pieces of 

 the greenish or brown compact interposition-bearing hornblende bordering 

 the aggregates or in the midst of them. The light-green reedy hornblende 

 never contains such interpositions, but does have associated with it fairly 

 large grains of rutile, which may perhaps be considered as having been 

 derived from the various titanium-bearing microlites in the original brown 

 hornblende. The general appearance of these aggregates and their asso- 

 ciation with the original hornblende seem to point toward their secondary 

 origin from the latter through the effects of pressure. 



Pyroxene. — The pyroxeue comprises both a monoclinic and an ortho- 

 rhombic kind. These are the first of the bisilicates to crystallize in these 

 gabbros. The monoclinic kind is of two varieties. The first is in colorless 

 to faint-pink grains included in large plates of original brown hornblende. 

 These grains have a well-developed prismatic cleavage. One basal section 

 shows very nicely the characteristic pyroxene cleavage. The extinction 

 measured against the prismatic cleavage reached as high as 50 degrees. 

 This pyroxene is presumed to be augite. It never shows diallagic parting. 

 In other sections the monoclinic pyroxene is a clear white to faintest 



