106 DR. J. W. EVANS ON THE KOCKS OF THE [Feb. I906, 



Hornblende is the principal and the only original, coloured con- 

 stituent. The crystals are usually very irregular in shape, often 

 branching. In many cases there is a border which apparently 

 differs in composition from the interior, the latter representing an 

 earlier period of crystallization. Sometimes the earlier crystal 

 appears to have been worn, or altered into chlorite or other 

 decomposition-products, before the later was deposited ; in other 

 cases, the centre appears to consist of flakes of hornblende, pseudo- 

 morphous after another mineral, perhaps augite. Occasionally the 

 outer and inner portions are in crystalline continuity with one 

 another, although the extinctions and other physical characters are 

 different ; but, in the majority of cases, the cleavage of the two 

 seems to stand in no definite relation, so that they appear to 

 be crystallographically independent, and yet the whole or the 

 greater part of the outer shell extinguishes together and forms part 

 of one crystal. 



The birefringence of the two portions is different. In one case 

 the rate of relative retardation in the section of the interior is 

 about 20 thousandths (*020), but is rather less in the neighbourhood 

 of the cleavage, where a certain amount of decomposition appears 

 to have set in ; while in the exterior hornblende the index is only 

 15 thousandths (-015). The angle of extinction with the vertical 

 cleavages is very small in the interior portions, while it is sometimes 

 as much as 32° in the exterior. The pleochroism is widely different, 

 the centre changing from greenish-yellow to yellowish-green, while 

 the outside shows different shades of bluish-green. 



Numerous inclusions of apatite occur in the hornblendes and 

 other porphyritic crystals. 



The remainder of the rock consists of a fine-grained ground- 

 mass. It occurs more particularly surrounding the porphyritic 

 quartz, and penetrating into its cavities. The chief constituent is 

 quartz in minute, idiomorphic, bipyramidal crystals, containing 

 occasional inclusions of apatite as well as small, colourless, isotropic, 

 negative crystals, probably of glass. Felspars are also numerous, 

 sometimes clear, and sometimes grey and cloudy from minute 

 inclusions. They rarely show signs of twin-lamellation, but their 

 refractive indices are usually well below that of Canada-balsam, 

 and they are probably in most cases orthoclase. Micropegmatitic 

 structure is visible, and in some cases it occurs as a border round 

 the porphyritic quartz-crystals, with which the pegmatitic quartz 

 is in crystalline continuity. Some comparatively-large grains of 

 magnetite are surrounded by hornblende, no doubt contem- 

 poraneous with the exterior zone of the porphyritic hornblende, 

 and there are areas where there is a poikilitic mixture of a pale- 

 green hornblende, showing slight but distinct pleochroism, and 

 felspar. In other places there are feathery aggregates of horn- 

 blende-microlites. Some ill-defined grey areas show a reddish 

 appearance by reflected light. 



The alteration of the interior of the porphyritic hornblendes and 

 the corrosion of the quartz seem to indicate that they were formed 



