Mr. J. A. PHILLIPS.^) It must be remembered, however, that at the time 

 when his description was written the modern methods of recognizing the 

 rhombic pyroxenes were not generally known and he therefore described 

 the pleochroic enstatite as hornblende. Rocks similar to those of the 

 Penmaenmawr quarries occur at Klausen in the Tyrol. They have been 

 investigated by TELLER and vox JOHX who have described them as norites 

 and quartz-norites. It seems desirable to limit the term norite to basic 

 rocks and we accordingly prefer to call the Penmaenmawr rock an 

 enstatite-diorite. In the first edition of his work on the massive rocks 

 ROSEXBUSCH referred to the Penmaenmawr rock as an enstatite-diabase 

 and in a paper on the Whin Sill the present writer described it as a bronzite- 

 diabase. Mr. PHILLIPS called the rock a quartziferous diorite. 



The least altered varieties consist essentially of felspar, enstatite, augite, 

 biotite, iron-ores and quartz. The dominant felspar is undoubtedly 

 plagioclase. Twinning on the albite plan is common and this is sometimes 

 associated with twinning on the Carlsbad plan. The common form of the 

 sections is lath-shaped. In the larger individuals the extinction is usually not 

 simultaneous for the whole of the section. The marginal portions extinguish 

 at lower angles than the central portions. Differences of 10 or 12 are not 

 uncommon. The two portions are not separated from each other by sharp 

 lines. The optical properties vary in the most gradual manner from the 

 centre towards the margin. These facts probably indicate that the larger 

 felspars vary in composition, the central parts being more nearly allied to 

 anorthite than the marginal parts of the same individual. In some slides 

 the plagioclase occurs in the form of tolerably large individuals and also 

 as small columnar crystals giving lath-shaped sections. Mr. WALLER ( ' 2 ) 

 observed six extinctions lying between 5-4 and 56 and accordingly 

 inferred the existence of labradoritc. Messrs. TELLER and VON JOHX 

 isolated and analysed the plagioclase of an allied rock from Klausen in 

 the Tyrol. Their analyses prove that in this case the felspar lies between 

 andesine and labradorite. 



In the Penmaenmawr rock the plagioclase is seen to be idiomorphic 

 with respect to the quartz as in so many other rocks of the diorite family. 

 The quartz indeed plays the role of ground-mass. It often occurs in 

 ophitic masses which possess uniform optic properties over considerable 

 areas in a microscopic slide. This interstitial quartz is, however, frequently 

 intergrown with an untwinned felspar which may with considerable confi- 

 dence be regarded as orthoclase. The micro-pegmatite so formed is 

 sometimes perfectly colourless. The felspar is then as clear and limpid as 

 the quartz. In such cases the only means of distinguishing one mineral 

 from the other in parallel polarized light depends upon the difference in 

 double-refractive power. This difference, though very slight, is often 

 decisive. To apply the test the sections should be of such thickness that 

 the quartz sections cut parallel to the optic axis polarize in the pale yellow 

 tints of the first order. This is the case with the slides as they are usually 



(1) Q.J.G.S. Vol. XXXIII. (1877), p. 423. 



(2) Midland Naturalist, 1885, p. 4. 



