H. J. Sei/mour — Bine Amphiboie in Hornblende. 259 



The biotite forms small (about 040 mm.) hypidiomorphic crystals 

 scattered fairly abundantly and evenly thi'ougbout the slide. Sections 

 at right angles to the cleavage planes are strongly dichroic (almost 

 colourless to greenish brown), and are frequently bent into curved 

 forms. Narrow bands of calcite are occasionally noticed between 

 the lamellae. The mineral is here and there altered to chlorite on 

 its edges, and also to a less extent along cleavage cracks. Sections 

 parallel to the basal plane give a uniaxial interference figure and 

 show an outer rim of a much darker colour than that of the interior 

 of the crystal. 



The felspars occur in medium-sized allotriomorphic crystals which 

 are rather altered and mostly micropoikilitic with numerous minute 

 inclusions. In their present condition the exact species is not 

 satisfactorily determinable, but repeated twinning is noticeable in 

 several of the less altered crystals, and they would seem to be 

 essentially plagioclase. 



Apatite is an abundant constituent in the rock, and occurs as 

 inclusions in the biotite, plagioclase, and in the green hornblende. 



This latter mineral is also hypidiomorphic, and appears to be 

 slightly posterior in date to the biotite, small crystals of which 

 it sometimes includes. Sections parallel to the principal axis attain 

 a length of 0-60 mm. The pleochroism is fairly strong, the tints 

 varying from yellowish to a dull dark green. Basal sections exhibit 

 the characteristic six-sided forms with well-defined cross cleavages 

 intersecting at an angle of about 124;°. The blue amphiboie occurs 

 mostly as a secondary addition to, but partly also as a replacement 

 of, the primary green hornblende, and has grown in crystallographic 

 continuity with it. The cleavage lines are usually continuous in 

 both varieties, but are finer and sharper in the blue amphiboie. In 

 most cases this latter is added to the clinopinacoidal or terminal 

 planes of the green variety, and occurs also, but less often, along 

 cracks in the interior of these crystals. The secondary enlargement 

 is quite limited in extent and seldom exceeds 0-10 mm. in greatest 

 length, the average being rather below this figure. The junction 

 line between the two varieties is generally distinct, and they are 

 also sharply marked off from one another by their notably different 

 pleochroism. Exceptionally a very narrow band or zone of material 

 of a much paler blue colour than the normal blue amphiboie occurs 

 between the latter and the green hornblende. The drawings show 

 the characteristic mode of occurrence of the secondary mineral in 

 question. Owing to its small size it does not present facilities for 

 satisfactorily determining its optical properties, but in so far as 

 it is possible to do so, they would appear to be as follows. Sections 

 approximately parallel to the clinopinacoid show that the extinction 

 angle is about 15°, and that this takes place on the opposite side 

 of the vertical axis to that of the green hornblende to which it has 

 been added. Observations with the quartz wedge appear to prove 

 that this direction of extinction corresponds also with the axis of 

 maximum elasticity. The pleochroism is strong and is as follows : 

 a =: sky-blue, b = pale reddish-violet, c = pale yellowish. The 

 absorption formula is a > b > c 



