PLATE 1 1 1. 

 FIG. i. 



AUGITE-PICRITE. 



SHIANT ISLES, WEST OF SCOTLAND. 

 Magnified 25 diameters. Ordinary light. 



The original minerals here represented are olivine (1), augite (7) and magnetite. The 

 secondary substances are serpentine after olivine (6), and a turbid material, which 

 gives aggregate polarisation under crossed nicols, and probably represents an alteration 

 product after felspar. 



The olivines occur in more or less rounded grains. Those represented in the lower 

 part of the figure have been partially converted into serpentine. 



The augite is a rich reddish brown colour in moderately thick sections. The 

 whole of this mineral represented in the figure extinguishes in one position as the stage 

 is rotated between crossed nicols ; thus showing that it belongs to one crystalline in- 

 dividual. It is entirely destitute of crystalline form. 



FIG. U. 



ANOTHER PORTION OF THE SAME SLIDE. 



Magnified 25 diameters. Nicols crossed. 



The minerals represented are olivine (1), plagioclase felspar, probably labradorite or 

 bytownite (10), and magnetite. 



The section is thick, so that the plagioclase gives brilliant colours of the first and 

 second orders instead of polarising in neutral tints as is the case in sections of the usual 

 thickness. 



The olivines either give the brilliant greens and pinks of the higher orders (some- 

 times of the fourth order), or else appear dark in consequence of being at or near the 

 position of extinction. 



The felspar shows the lamellation characteristic of twinning on the albite plan. It 

 will be observed that the felspar occurs in the form of large plates without any definite 

 external faces, and that a large number of olivines are included in one and the same 

 felspar individual. The rock is holo crystalline, and both the felspar and augite form, as 

 it were, the groundmass in which the crystals and grains of olivine are embedded. This 

 rock is one of the series described by PROF. JUDD in his paper on the " Tertiary and 

 Older Peridotites of Scotland." (Q.J.G.S., Vol. XLI., p. 393). 



