﻿478 ME. J. A. THOMSON ON THE HOENBLENDIC [NoV. I908, 



In sections the minerals that present themselves are apatite, 

 magnetite, pyrite, olivine, augite, amphiboles of various colours, 

 chlorite, and calcite. The augite and olivine are included 

 poecilitically in large plates of hornblende. 



Apatite is a very rare constituent, and is generally semi- 

 opaque with fine inclusions. 



The olivine has often the appearance of being remarkably 

 fresh, but has really been destroyed in part. It has a slight reddish- 

 brown hue and a higher relief than is usual, both these characters 

 heing due to a very fine schiller-structure. Examination under high 

 powers reveals the presence of numerous, small, opaque inclusions 

 arranged in parallel rows. The crystals sometimes show a tendency 

 to idiomorphism, and the rows of inclusions are then parallel to 

 the elongation. Where the crystal-form has been destroyed, the 

 extinction parallel to the rows shows that they are arranged in a 

 pinacoidal plane of the crystal. The olivine is penetrated by the 

 usual cracks. It never shows any phenomena of serpentinization 

 along the borders or cracks, but the latter are penetrated by fibres 

 of chlorite. At the junctions of the cracks there are often rounded 

 blotches of pyrite. 



The augite is a colourless variety, with little trace of schiller- 

 structure. The crystals are of small size, and are often twinned on 

 (100). They are rarely idiomorphic. Generally they are included 

 in hornblende, and are then, as a rule, bordered by a zone of 

 magnetite-dust of variable thickness. The relations are suggestive 

 of magmatic resorption of the augite, with formation of hornblende. 



Several varieties of amphibole may be distinguished by colour, 

 even within the same crystal ; but a green hornblende is pre- 

 dominant. It forms large plates, seldom with regular boundaries, 

 although small cross-sections of clearly-cut prisms are found. It is 

 not markedly pleochroic in green and yellowish tones : — X pale 

 yellow to colourless, Y yellow-green, Z bluish-green. The bi- 

 refringence is fairly strong. The varieties in colour diverge in two 

 directions, to a brownish hornblende and to a colourless tremolite. 



The tremolite is often outgrown on the green hornblende in 

 crystallographic continuity, the boundary sometimes appearing to 

 correspond with crystallographic faces. The tremolite, as usual, 

 shows higher interference-colours. There is, in addition, a con- 

 siderable amount of tremolite occurring independently — for the most 

 part in the form of plates, which resolve themselves under crossed 

 nicols into irregular fibrous aggregates. 



The brown hornblende is generally found in large plates, 

 which show a transition to green. The pleochroism is : X brown, 

 Y yellow-brown, Z brown with a lilac tinge. It is a common 

 brown hornblende and not basaltic hornblende.^ It seems probable 



1 Dr. Weinschenk (' Die Gesteinsbildenden Mineralien ' Freiburg i. B., 

 1901, p. 105) distinguishes between common brown hornblende with Httle or 

 no titanic acid and extinctions up to 18°, and basaltic hornblende with titanic 

 acid up to 5 per cent, and extinctions up to 10°. 



