P LA T E XI 



FIG. I 



DIABASE. 



SIDE OF CLOCHNANT : 1^ in. E.S.E. OF BLAENVDKE. 

 Magnified 25 Diameters. Ordinary light. 



The minerals represented are augitc (7), hornblende (8), magnetite or ilmenite. 

 serpentine (6) and pseudomorphs after felspar (lOa). 



The augite is present in large ophitic plates. Hornblende occurs sparingly 

 and appears to be intergrown with the augite. The serpentine is yellow and 

 similar to that which commonly results from the alteration of divine, except that 

 secondary iron-oxides are rare. 



The felspar is mostly represented by a turbid substance giving aggregate 

 polarisation. Sometimes traces of twin lamellation may be detected. 



The rock was originally an ophitic dolerite. It is associated with strata of 

 Lower Bala age. (See Q.J.G.S., vol. XXXVI, p. 279). 



FIG. II. 



OLIYINE-DOLERITE. 

 ROWLEY HILLS, STAFFORDSHIRE. 



Magnified 50 Diameters. Ordinary light. 



The minerals represented are olivine (1), augite (7), felspar (10), iron-ores and 

 serpentine. Colourless needles of apatite and a small amount of isotropic interstitial 

 matter also occur. 



The large olivine in the upper part of the figure is not an individual, but an 

 aggregate of seven or eight grains. Serpentinisation has taken place along the 

 boundaries of the different grains and in one or two cases along cracks traversing 

 individual grains. The olivine is distinctly coloured, even in very thin sections, 

 and as the colour approximates to that of hyalosiderite and fayalite it seems 

 reasonable to infer that it is rich in iron. 



Augite occurs in small crystals, grains and granular aggregates : not in largp 

 ophitic plates. It is brown in colour. 



Felspar plays the role of groinulmass in the rock and occurs in forms giving 

 lath-shaped sections and also in more or less irregular grains or plates. 



A small quantity of glassy or micro-felsitic matter occurs here and there in 

 the interstices between the felspars. 



The rock is associated with Carboniferous strata. It lias been described by 

 Mr. ALLPORT (Q.J.G.S., vol. XXX., p. 54S). 



