MM 



the felspar is perfectly fresh ; here and there, however, a granular alteration 

 product has been developed. This is sometimes scattered irregularly 

 through the felspar substance and sometimes limited to the twin-pianos. 

 The biotite occurs sparingly and is generally intergrown with, or included 

 in the hornblende. Sections at right angles to the cleavage change from 

 a pale yellowish brown to a rich green as the stage is rotated. The 

 original mineral has evidently been more or less altered. That the gi\vn 

 mineral is not typical chlorite is proved by the fact that it possesses 

 tolerably high double-refractive power. The quartz occurs in irregular 

 grains. It plays the role of ground-mass as in the Arran rock already 

 described and as in the augite-diorites (quartz- gabbros) of Carrock Fell. 

 Apatite occurs in stout prisms. Sphene occurs as irregular reddish brown 

 grains and also in the form of narrow zones surrounding the opaque iron- 

 ores. It is distinguished by its high refractive and double-refractive power. 

 The relation of the sphene to the iron-ore is not such as to imply that the 

 former mineral has been produced at the expense of the latter. The 

 sphene appeal's to have been developed on a nucleus of iron-ore. 



The dark-coloured rock is composed of brown and green hornblende 

 more or less turbid felspar, green biotite. quartz, iron-ores, apatite and 

 epidote. The hornblende is often of a deep brown colour, similar to that 

 of the hornblende of the picrites and oliviue-diorites. It is often pene- 

 trated by lath-shaped felspar sections. Quartz and felspar are much less 

 abundant than in the previous specimen. Sphene is not recognizable in 

 the section examined. 



Professor VON LASAULX W has described quartz-diorites and quartz- 

 niica-diorites from the lower Palaeozoic district of Co. Wieklow. A rock 

 from Bennlettery (quartz-diorite) is described as consisting of plagioelase 

 often much altered, hornblende, quartz, epidote, viridite, apatite and opaque 

 iron-ores often bordered with titanomorphite [sphene]. A rock from Pini- 

 ganstown, east of Rathdrum, is described as a miea-diorite. It consists of 

 plagioelase, hornblende, viridite. mica, quartz, magnetite, ealeite. pyrite, 

 epidote and ferric oxide. The plagioelase is much altered and shows only 

 aggregate polarization. The hornblende is largely replaced by viridite. 

 Epidote ocelli's in yellowish pleochroie grains associated with the viridite. 

 Mica is the most abundant constituent next to plagioelase. Basal sections 

 are reddish brown in colour in the centre and green at the margins. 

 Sections at right angles to the principal cleavage show intense pleochroism, 

 changing from light yellow to dark brown as the stage is rotated over the 

 polarizer. Basal sections examined in convergent light show the inter- 

 ference figure of a biaxial mineral with very small optic axial angle. 

 Magnetite is scattered through the rock in the form of octahedra. 



Specimens of some of these Irish diorites are present in the ALLPORT 

 collection in the British Museum. A slide of the quartz-mica-diorite from 

 Punganstown agrees substantially with the description of vox LASAULX. 

 The hornblende is variable in tint as in the Glen Tilt rocks. The 



(1) T.M.M., Neue Folge, Vol. I., p. 443 r 



