CHARACTERS OF THE DIABASE. 387 



The uralite in these slides is largely light-brownish in color, with a slight 

 greenish tinge, but is plainly dichroic or polychroic ; it is distinctly and 

 finely fibrous, and usually extinguishes a few degrees from the direction of 

 the fibers. Cross-sections of the prisms show the crystal form of augite, and 

 in places it is plainly seen to be derived from that mineral. 



The color of the C ray of the uralite is pale greenish-brown, and that of 

 the A ray pale brownish. The interference colors of the uralite are largely 

 purplish. There is about as much uralite as augite. The frayed ends of 

 the uralite prisms sometimes pass over into chlorite. Some of the uralite is 

 stained with ferrotite. 



The chlorite is plainly pleochroic, bluish-green when the fibres are parallel 

 to the shorter diagonal of the polarizer, and pale brownish when at right 

 angles. The polarization tints are deep-brown and indigo-blue, sometimes 

 so dark that no light passes through. The chlorite is frequently derived 

 from the uralite, but sometimes seems to be formed directly from the augite. 

 In some cases it fills the spaces between the other constituents. 



Ilmenite showing a grate-like structure occurs in much of the diabase. 

 Considerable portions of the diabase area, how'ever, contain no augite, the 

 bisilicate being hornblende. Such portions, therefore, are practically diorite. 

 The ridge of which Black point is a })art is, from that peak southward to 

 the serpentine dike, largely of this diorite. The rock of Eagle point ridge, 

 immediately north of the same serpentine dike, is also largely hornblendic. 



Number 307 is a typical example of this diorite. It comes from the 

 southern slope of Black point. This rock, viewed macroscopically, is dark 

 in color, very fresh looking, and of even texture. Other specimens have a 

 much coarser texture, with more abundant feldspars, giving a grayish color, 

 then more resembling ordinary eruptive diorite as to the macroscopic appear- 

 ance. The hornblende of number 307 is fibrous, usually greenish, and 

 contains numerous granules of iron oxide. No augite was noted in the slide. 

 The plagioclase is fresh looking, polysynthetic and lath-like, largely im- 

 bedded in the hornblende. 



In some other slides of the diorite there are no granules of iron oxide in 

 the hornblende, and it is then indistinguishable from the light greenish- 

 brown uralite of slides 43 and 143. 



Slide number 283 shows some augites, minute scattered patches of which 

 have been converted into uralite. Some of the hornblende of the slide is 

 compact, all portions of the crystal extinguishing at once ; and such prisms 

 are not so evidently fibrous, but show prismatic cleavage plainly. Minute 

 patches of some of this brown compact hornblende are decidedly bluish in 

 color, suggesting glaucophane. Greenish fibrous hornblende is sometimes 

 associated in the same prism with the brown compact variety, suggesting 

 the formation of one from the other ; in other prisms of the compact horn- 



