PLATE XXI. 



FIG. 1. 

 HOENBLENDE-SCHIST. 



MAKGIN OF SCOURIE DYKE. 



Magnified 100 diameters. Polariser only ; short axis N. & S. 



The minerals represented are hornblende (8), felspar (10), and iron-ore (5). 



This rock was taken from the margin of the dyke and is of finer grain than 

 that taken from the centre. The structure is that of a typical crystalline schist. 

 The felspar is colourless, and occurs in irregular grains. It is for the most part 

 untwinned. Some quartz may be associated with it, but most of those grains 

 which are cut in a direction favourable for examination with convergent light 

 give the figure of a biaxial crystal. 



This figure completes the series illustrating the passage of a massive plagio- 

 clase-pyroxene rock into a typical hornblende-schist. 



FIG. 2. 

 DOLEBITE NEAE CONTACT WITH GEANITE. 



NEAR BOVEY TRACE Y, DEVONSHIRE. 



Magnified 50 diameters. 



The minerals represented arc augite (7), hornblende (8), brown mica (9), 

 felspar-mosaic (10) and opaque iron ores. 



The felspar-mosaic was drawn under crossed nicols. The other constituents 

 are represented as they appear in ordinary light. 



The upper portion of the figure shows a somewhat altered crystal of augite 

 with traces of the original prismatic cleavage. This has been altered at its 

 margin into hornblende. The original felspar is now represented by an aggregate 

 of colourless grains. That this aggregate is secondary is proved by the fact 

 that it contains needles and grains of the secondary hornblende. The brown 

 mica belongs to the variety which is characteristic of contact alteration. 



