P LAT E VI. 

 FIG. I. 



HORNBLENDE-PICRITE (BONNEY.) 

 TY CUOES, ANGLESEA. 



Magnified 50 Diameters. Ordinary light. 



The two figures on this plate are intended to illustrate some of the characters 

 commonly observed in the hornblende of this group of rocks. 



The portion of the slide here represented is composed of hornblende in various 

 conditions (8), a colourless pyroxene (7) and a colourless substance forming a sort 

 of groundmass and composed of an aggregate of fibres and scales which give a definite, 

 though not very strong reaction with polarised light. 



Near the central portion of the figure (8 a ) is a cross-section of a hornblende-prism. 

 Portions of it have a decided brownish tint not represented in the figure. These 

 portions shade into others which are green, and these again into others which are 

 perfectly colourless. In the lower part of the figure to the right two vertical sections 

 of hornblende (8 b ) are represented. These also illustrate .the transition from the 

 green to the colourless variety. The colourless portions, moreover, are here seen to 

 shade off into the colourless substance which forms, as it were, the groundmass. 

 Under crossed nicols a considerable amount of calcite dust may be recognised at 

 the points where the hornblende loses its individuality. 



In other portions of the slide a colourless pyroxene may be recognised. The only 

 section of this mineral in the figure (7) occurs as an inclusion in the hornblende on 

 the lower right-hand margin. 



FIG. II. 

 The same under crossed nicols. 



The change in the colour of the hornblende is here seen to be accompanied by a 

 rise in the order of the tint given under crossed nicols. The indigo of the second 

 order shades gradually into the yellow of the same order. This of course indicates an 

 increase in the double-refracting power of the mineral. The different portions of the 

 hornblendes (8 b i extinguish simultaneously at an angle of 2'2 . The colourless substance 

 into which the hornblende-fibres fade away sometimes contains brilliantly 

 polarising fibres (c). Calcite dust (d) occurs in patches in different parts of the 

 slide. 



The rock is described by I'nor. BONNEY (Q.J.G.S., Vol. XXXIX., p. 254). 



