PLATE I. 

 FIG. i. 



LHERZOLITE. 



SEM NEAR VICDESSOS, PYRENEES. 



Magnified 25 Diameters. Ordinary light. 



The essential constituents of this rock are olivine (1), enstatite (2), chrome- 

 diopside (3), and chrome-spinelle or pleonaste (4). 



In the perfectly unaltered rock here figured it is sometimes difficult to distinguish 

 between olivine and enstatite. Both minerals are perfectly colourless; but, as a rule, 

 the olivine sections give higher tints in Newton's scale, when viewed under crossed 

 nicols, than the enstatite. Where cleavages have been developed, as in the crystalline 

 grain represented in the upper part of the figure to the left, the enstatite may be 

 readily distinguished by its optical properties. The olivine forms about two-thirds of the 

 entire rock. It is traversed by irregular cracks and bands of inclusions as shown in the 

 figure. Under a high power the inclusions are seen to possess a peculiar vermicular form. 

 The chrome-diopside may be distinguished in the original slide by a very faint greenish 

 tinge which, however, is not represented in the figure, and also by its cleavages and 

 optical properties. 



The dull green mineral is chromite, picotite or possibly pleonaste. Chromite and 

 picotite, the common minerals of this group occurring in the peridotites, are usually 

 yellow, brown or reddish-brown. Pleonaste is green. The colour may therefore indicate 

 that in the case here figured the mineral in question is pleonaste. The rock is holo- 

 crystalline and granitic in texture. The individual minerals do not show definite 

 external form. 



FIG. II. 

 SERPENTINE. 



BETWEEN KYNANCE COVE AND LIZARD TOWN, CORNWALL. 



Magnified 25 diameters. Ordinary light. 



In addition to the serpentine-substance, wliich is either green or colourless, un- 

 altered grains of olivine (1), and a crystal representing the bastite-modification of 

 enstatite or bronzite (2) may be detected. 



The alteration of the olivine has taken place along the irregular cracks wliich 

 traversed the original grains. The brown colour represents the hydrated ferric oxide 

 which has been precipitated along these cracks by oxidising agents. The irregular net- 

 work (maschen-structur), characteristic of serpentine produced by the alteration of 

 olivine, is well represented in the lower and upper parts of the figure by the distribution 

 of the ferric oxide. 



