306 LADY McROBERT ON ACID AND OTHER [Dec. 1914, 



It is composed of a fine-grained, pink, orthophyric trachyte 

 rich in soda. The phenocrysts are chiefly orthoclase, or twinned 

 albite with a vein of orthoclase, and some soda-orthoclase. The 

 ground-mass is mainly of orthophyric orthoclase embedded in 

 limonite, which replaces the original ferromagnesian mineral, 

 possibly riebeckite. 



The Black Hill, with its steep southern slopes of platy 

 screes, is an intrusive sheet of porphyritic quartz-riebeckite- 

 tracr^te, this expression being used to denote a porphyritic 

 trachyte which contains a fair proportion of interstitial quartz. 

 This quartz occurs most freely on the upper part of the sheet. 



On the south side of the hill the lower contact of the trachyte 

 with Upper Old Red Sandstone containing scales of Holoptychius 

 is well seen in a disused quarry. It runs nearly parallel with the 

 bedding of the sandstone, but cuts across it in places. At the 

 back of the hill the Old Red Sandstone overlies the trachyte, 

 thus forming the roof of the sheet. The rock is sometimes 

 beautifully banded parallel to its floor, which dips gently northwards. 

 In the quarry the trachyte is compact and non-porphyritic at its 

 base ; but within a foot above the junction it shows scattered 

 phenocrysts of white sanidine, which increase in number towards 

 the more massive centre of the sheet. The rock is characterized 

 by a platy, crinkly fluxion-cleavage. The best-marked joints are 

 parallel to the dip. When struck with a hammer it emits a 

 peculiar sulphurous smell. The mineral constituents are : — 



Sanidine, as fresh phenocrysts, and laths showing Carlsbad twinning in 

 the ground-mass. 



Quartz, abundant interstitially. 



Limonite, filling spaces among the felspars ; in one slide are bluish- 

 green pleochroic needles among the secondary limonite, and these 

 have the habit of the Eildon riebeckite. 



The structure is trachytic. 



There is a lower sheet in the wood above the road. 



Bemerside Hill has been cut into by the Tweed so as to 

 form a steep cliff of non-porphyritic quartz-trachyte (see 

 PL XLI, fig. 5). The contacts of the trachyte are everywhere 

 much covered with grass, but the appearances rather suggest a 

 dome-shaped massive intrusion. In the quarry near the turn 

 in the road a well-jointed face is exposed, with one set of joints 

 arching roughly parallel to the outline of the hill, which looks 

 as though its present surface approximately corresponds with the 

 original form of the intrusion. The other set of joints are about 

 15° from the vertical near the edge, and become vertical near 

 the centre. The rock is purplish and compact, with pinkish- white 

 spots and a few scattered felspar-phenocvvsts. It weathers yellow, 

 showing a considerable development of white talc (or kaolin) and 

 pyrolusite along the cracks. Under the microscope it is seen to be 

 a, mass of trachytic sanidine-needles, with interstitial quartz and 

 much limonite. 



