PLATE vui. 



FIG. I. 

 HORNBLENDIC DIABASE (TAWNEY). CAKBO LLEFAIN, CAERNARVONSHIRE. 



Magnified 25 diameters. Drawn with polariser only ; short diameter of Nicol right 



and left. 



The minerals represented are hornblende (8), augite (7), felspar (10), magnetite and 

 viridite (13). 



The hornblende is the usual brown variety. It occurs in large irregular plates, in 

 which other minerals are frequently enclosed. 



The augite is nearly colourless. It is intergrown with, and sometimes enclosed 

 in the hornblende. 



The felspar occurs in lath-shaped sections, and also in irregular plates. It is 

 frequently traversed by cracks which have been filled with viridite, as is represented in 

 the lower part of the figure near the margin to the right. The felspar itself is often 

 perfectly unaltered, and shows lamellar twinning. In the upper part of the figure (10A) 

 there is a cloudy patch giving aggregate polarisation which probably represents an 

 alteration product after felspar. 



Viridite is not used as the name of a definite mineral species ; but only for those 

 green alteration products which cannot be definitely referred either to chlorite or 

 serpentine. Some of the viridite in the slide from which the figure is taken certainly 

 arises from the alteration of hornblende ; but the portions represented in the figure are 

 suggestive of olivine. 



The rock forms a part of the Peimrfynydd ridge, and is apparently intrusive in 

 Lower Arenig strata. It has been described by Mr. TAWNEY (G.M., 1880, p. 211). 



FIG. II. 



JUNCTION OF TKOKTOLITE AND SERPENTINE, COVERACK COVE, CORNWALL. 

 Magnified 25 diameters. Ordinary light. 



The minerals represented are olivine (1), plagtoclase (10), iron-oxides and serpentine. 



The section is cut so as to show the junction between the troktolite (anorthite- 

 olivine rock) and serpentine. The troktolite is intrusive in the serpentine according to 

 Professor BONNEY. 



The right-hand third of the figure represents the serpentine ; the left-hand two- 

 thirds, the troktolite. The serpentine has been formed by the alteration of an olivine 

 rock, and grains of unaltered olivine still remain. It has been stained along the cracks 

 by the separation of ferric oxide. 



The olivine of the troktolite has been changed into a precisely similar serpentine. 



The plagioclase of the troktolite, though much traversed by cracks, is remarkably 

 fresh. It occurs in large irregular grains and coarse granular aggregates. The 

 individuals frequently show, under crossed nicols, a broad lamellar twinning. 



In other portions of the slide the cracks in the felspars are seen to radiate from the 

 altered oli vines, as though they had been formed by mechanical forces due to an 

 expansion of the olivine consequent on serpentinisation. The twin lamella? of the 

 felspar are often bent ; a fact which also proves that the crystals have been strained. 



The rock has been described by Professor BONNEY (Q.J.G.S., vol. XXXIII., 

 p. 906 ; also G.M., 1885, p. 442). 



