PLATE xv. 

 FIG. i. 



OLIVINE-HORNBLENDE SERPENTINE. 



KYNANCE, THE LIZARD, CORNWALL. 

 Magnified 25 diameters. Ordinary light. 



The substances represented in this figure are olivine (1), hornblende (8) and 

 serpentine stained with ferric oxide. 



The olivine is traversed by the usual network of serpentine veins, portions of 

 the unaltered mineral remaining as minute granules in the centres of the meshes. The 

 serpentine substance is stained with ferric oxide. The outlines of the original olivine 

 grains are indicated, \inder crossed nicols, by the areas over which the detached grains 

 give simultaneous extinction. 



The hornblende is pale green in colour and without any definite crystalline form. 

 It is comparatively unaltered, and does not show any marked pleochroism in thin 

 section. The grains are elongated in the direction of the vertical axis, and are 

 arranged in the rock with their long axes roughly parallel to each other. The 

 cleavages are characteristic and the maximum extinction in the prismatic zone (i.e. in 

 the zone in which the cleavage cracks are always parallel), is about 20. The variety 

 of hornblende here represented appears to be identical with that named smaragdite by 

 BROGGER in his description of the Almeklovdal peridotites. The original rock must 

 have been an olivine-hornblende peridotite allied to many of the Norwegian peridotites. 

 (See Professor BONNEY, Q.J.G.S., Vol. XXXIX., p. 28.) 



FIG. II. 

 OLIVINE-HORNBLENDE SERPENTINE. 



MULLION COVE, CORNWALL. 

 Magnified 40 diameters. Ordinary light. 



The minerals represented are olivine, hornblende, serpentine and magnetite. 



This rock is distinctly banded. The lower portion of the figure represents a band 

 extremely rich in hornblende, the upper portion one extremely rich in olivine. 

 The dividing line runs upward from left to right. The principal interest of 

 the figure lies in the fact that it furnishes evidence of the formation of serpentine 

 from hornblende, as well as olivine. This is well seen near the spot marked 8a. A 

 mass of hornblende is here seen to be broken up at its margins by serpentine veins. 



The secondary iron oxides are in the condition of magnetite, and not in the 

 condition of ferric oxide, as was the case in the last described rock. 



