62 A DESCRIPTION OF THE [Ch. V. 



In the former modification, quartz-rock becomes tinged 

 throughout its mass of a yellowish, greenish, or purplish 

 colour, by an infusion of the matter of clay-slate, and passes 

 occasionally even into hornstone and flinty slate. As the 

 proportion of clay-slate increases, the compound becomes 

 entitled to the name of quartzy clay-slate ; and at last it passes 

 into pure clay-slate, both compact and fissile. In the beds, 

 in which quartz predominates, scales of mica sometimes occur, 

 and are disposed without order through their mass. In the 

 slaty beds, however, they appear frequently arranged in the 

 direction of the laminae, silvering over the planes of separ- 

 ation. Quartz-rock, impregnated with the matter of clay- 

 slate, frequently envelopes grains of quartz, scales of mica, 

 sometimes, also, grains of felspar, and rarely minute portions 

 of clay-slate; thus constituting a firm compact variety of 

 greywacke\ When clay-slate matter predominates, it passes 

 into greywacke -slate. All these varieties of quartz-rock are 

 more or less traversed by small contemporaneous veins and 

 strings of pure white quartz, which in their range frequently 

 follow the line of the dip." * 



" The greenstone, as at Dunganstown Hill, is near the sum- 

 mit more crystalline than in other parts, composed of distinct 

 crystals of felspar and hornblende ; but in general hornblende 

 appears only in spots, or as colouring the felspar mass, which 

 is partly compact and partly foliated, and in which are dis- 

 seminated minute crystals of felspar. The grain varies from 

 fine granular to compact ; and the common character of the 

 rock in the latter state is that of a greyish green base with an 

 uneven fracture, which contains disseminated minute crystals 

 of glassy felspar. In the north-eastern part of this hill the 

 greenstone approaches more to the nature of an ash-grey com- 

 pact felspar, partly vesicular, the vesicles being mostly empty, 

 or slightly coated with iron ochre, derived from the decom- 

 position of iron pyrites, which is present in these trap rocks." f 



* Geol. Trans., vol. v. p. 185. f Idem, p. 170. 



