PLATE i v. 



FIG. I. 



HORNBLENDE-PICRITE (BONNEY). BOULDER, PEN-Y-CNWC, ANGLESEA. 

 Magnified 50 diameters. Ordinary light. 



The portion of the slide represented in the figure is mainly occupied by a single 

 individual of brown hornblende (8). The traces of the two sets of cleavage planes are 

 nearly parallel, showing that the section lies approximately in the zone of the vertical 

 axis. One important feature of the hornblende of this slide is that certain portions of it 

 are green or colourless (8 a ). Under crossed nicols the brown portion represented in the 

 figure gives the blue of the second order, whereas the colourless portion gives the yellow 

 or orange of the same order. In other portions of the slide a perfectly gradual transi- 

 tion from the brown through the green to the colourless variety may be observed ; and 

 in such cases the interference colours under crossed nicols are seen to rise in 

 NEWTON'S scale as the colourless condition is approached. The positions of extinction 

 are not affected by the change in colour. 



The greenish patches (6) represent serpentinous pseudomorphs. 



The black grains are somewhat translucent at the edges, and therefore probably 

 belong to chromite or picotite. 



The rock from which this slide is taken is described by PROF. BONNEY (Q.J.G.S., 

 Vol. XXXIX. p. 254). 



FIG. II. AUGITE-PICRITE. INCHCOLM, FIRTH OF FORTH. 

 Magnified 25 diameters. Ordinary light. 



The original minerals are olivine (1), augite (7), biotite (9), hornblende (8), 

 felspar (10) and magnetite or ilmenite (5). The secondary substances are yellow and 

 green serpentine (6), and alteration products after felspar (11). 



Olivine is by far the most abundant mineral. It occurs in the form of well marked 

 crystals, and also as grains. In some varieties of the rock it is entirely changed to 

 yellow or green serpentine. 



The augite is usually a pale reddish brown by transmitted light. A cross 

 section of an augite prism, similar to the one represented in the central portion of the 

 figure, always shows an optic axis when viewed with convergent polarised light. The 

 corresponding section of a rhombic pyroxene shows a positive bisectrix. 



Sections of the biotite parallel with the vertical axis of the crystal change in tint 

 as the stage is rotated over the polariser from a deep rich brown when the short axis of 

 the nicol lies parallel with cleavages, to a pale brown when the short axis is at right 

 angles to the cleavages. Basal sections show, of course, no pleochroism. 



The hornblende is the rich brown variety commonly known as " basaltic hornblende." 

 It occurs on the borders of the augite, as may be seen in the crystal on the right-hand 

 margin of the figure only part of which is included within the field of view. 



Plagioclase felspar is present only in very subordinate quantity, and in some 

 varieties of the rock appears to be entirely absent. Unaltered felspar is rare, but 

 a small portion is represented near the left-hand margin of the figure. The turbid 

 aggregate near the central portion of the figure is probably due to the alteration of 

 felspar. Some of the grains of this aggregate give brilliant colours under crossed 

 nicols, and probably belong to a mineral of the epidote group. 



The right-hand portion of the augite near the centre of the figure is penetrated by 

 a lath-shaped section of a pseudomorph after felspar ; thus showing that the rock is 

 allied to the ophitic olivine-dolerites, and belongs to the same type as the rock from 

 Menhenipt, (Plate II., Fig. 2). 



The rock was first described by Dr. A. GEIKIE (Trans. Roy. Soc., Edinburgh. 

 Vol. XXIX., 1880, pp. 507, 508). 



