ART. 2. PETROLOGY AT GOOSE CEEEK SHANNON. 49 



persion is very strong, r<v. Extinction highly inclined, about 45° 

 in one direction and parallel in the other. 



HORNBLENDE-FILLED CRACKS WITHOUT ALTERATION OF THE ADJACENT DIABASE. 



In the central part of the quarry there are frequently seen broad 

 plane surfaces veneered with glistening cleavages of a black mineral 

 up to 2 by 3 mm. in size associated with a little biotite and chalcopy- 

 rite These are found to be old joint fractures which have been healed 

 by later coarsely crystalline hornblende. Splitting tends to take 

 place along the hornblende filling and leave broad surfaces exposed. 

 Cross sections of these cracks show only a very narrow black line 

 with no alteration of the wall rock. 



A cross section cut from one of these showed the hornblende to 

 be entirely a filling of the open space of the crack and the constitu- 

 ent minerals of the rock are entirely unaffected by an}^ alteration. 

 Where an augite blade has been broken in two by the crack, the in- 

 tervening space is filled in by hornblende in parallel position. While 

 most of the hornblende is of the dirty brown color, an occasional one 

 is found tipped with blue hornblende in optical continuity. 



The glistening black hornblende was examined in powder under 

 the microscope and found to be biaxial negative with 2y medium. 

 The indices of refi action are a= 1.660, /3= 1.671, 7= 1.688, Birefring- 

 ence 7 — a =.028. The mineral is strongly pleochroic, with X=pale 

 greenish brown, Y = deep greenish brown, Z = deep brownish green. 

 The absorption is Z > Y > X. The optical orientation is Y = & and the 

 maximum extinction Zac= 16°. 



BLUISH HORNBLENDE COATINGS ON FRACTURE SURFACES. 



As has been noted under the discussion of jointing and fissuring, 

 the breaks which characterize the east-west fractures are surfaced 

 with a bluish gray coating, usually somewhat slickensided but easily 

 distinguishable from the glossy black ''diabantite varnish" of the 

 north-south joints discussed under a later caption. This bluish 

 material is mainly hornblende, although a small proportion of a light 

 green chlorite also occurs. The best example of this hornblende 

 coating seen is about 1 mm. thick and is pale blue-green with a silky 

 luster and a peculiarly ribbed surface, recalling the ripple marks on 

 shallow water deposited sediments. 



Under the microscope the material of this coating is found to be 

 finely fibrous with positive elongation, the extinction being about 13° 

 maximum. It is pleochroic in brownish blue-green across the length 

 and grayish blue-green parallel to the length. The indices of refrac- 

 tion are a= 1.630, /3= 1.642, 7= 1.650. It is obviously a hornblende. 



