PLATE vn. 



FIGS. I., II., III., and IV. 

 AUGITE-PICRITE. INCHCOLM. 



These figures illustrate some of the characters presented by the augite and 

 hornblende of this rock. 



Figs. I. and II. show a section of augite with peripheral hornblende. It is 

 taken out of the zone 100 : 001 and shows an optic axis when examined with 

 convergent polarised light. Fig. I. is drawn with the short diameter of the polarising 

 nicol (analyzer removed) parallel with the length of the crystal-section ; Fig. II. 

 with the short axis at right angles to the length of the crystal-section. The augite 

 is seen to be markedly pleochroic. It must be remembered, however, that as the 

 section is approximately at right angles to an optic axis it is not cut in such a 

 direction as to contain two of the axes of elasticity. The central portion of the augite 

 is nearly colourless and the colour of the external portion increases somewhat towards 

 the margin. The hornblende is remarkable on account of the intensity of its 

 pleochroism. The boundary between the augite and hornblende is often very 

 irregular : flecks and irregular patches of the latter mineral penetrate into 

 the former near the zone of contact. 



Figs. III. and IV. are taken from a section parallel to the vertical axis of both 

 crystals. They show the " hour-glass " structure of the augite. Fig. III. is drawn 

 with the polariser only, the short diameter of the nicol being parallel with the vertical 

 axis of the crystal. Fig. IV. is drawn with polarised light ; the nicols being 

 arranged in such a way as to bring out strongly the " hour-glass " structure. 



The interior and colourless portion of the augite extinguishes at an angle of 

 39 ; the external and coloured portion at an angle of 47. The section is therefore 

 taken approximately parallel to the clinopinacoid (010). 



FIG. V. 

 HORNBLENDE-PICRITE. 



PENARFYNYDD, ANGLESEA. 



Magnified 25 diameters. Drawn with polariser only ; short diameter of nicol 



right and left. 



The minerals represented are olivine (1), hornblende (8), chlorite (12), magnetite 

 and serpentine. 



The olivine is colourless. It contains inclusions of magnetite and is traversed 

 by narrow anastomosing strings of serpentine. The hornblende is mainly composed 

 of the brown variety, but it contains here and there small colourless patches. It 

 contains also strings and patches of dusty looking magnetite. The chlorite appears 

 to result from the alteration of biotite. Its mode of occurrence and optical characters 

 are described in the text. 



This rock was first described by Professor BONNEY (G.M., 1880, p. 208). It 

 is probably intrusive in Lower Arenig strata. 



