Vol. 70.] INTRUSIONS NEAE MELROSE. 309 



has yielded a profusion of platy screes. In the lower layer the 

 rock is pink, with small dark patches of riebeckite. Under the 

 microscope the riebeckite-growths are minute, and largely altered 

 to limonite. In the upper layers the rock is purplish grey, 

 weathering pale pink or white, with conchoidal fracture. Fresh 

 riebeckite is abundant in nests and irregular aggregates : it is 

 pleochroic. from deep blue to lemon-yellow. The ground-mass is 

 felsitic, with much secondary quartz in large plates. The higher 

 parts of the upper layer become coarser, and show microporphyritic 

 sanidine-ciystals ; a brownish-green biotite also occurs in one slide 

 from the summit of the Wester Hill. 



This rock, as indicated later, is the only one containing fresh 

 riebeckite that is found in any of the necks. In these higher 

 horizons there is an increasing proportion of soda, and phenocrysts 

 of soda-orthoclase have been found in the corresponding rocks of 

 the Mid Hill. These rocks closely approach keratophyres. 



Two interesting rocks occur towards the top of the Mid Hill. 

 The actual summit consists of orthophyric riebeckite - 

 trachyte, while to the west of this occurs augite-olivine- 

 trachyte. Hand-specimens from the junction can be selected 

 showing the two types intimately mixed. 



The orthophyric riebeckite -trachyte (PI. XLI, fig. -I) 

 is a very hard, compact, brown rock with a contorted fluxion- 

 cleavage, recalling in appearance corrugated iron. Riebeckite is 

 conspicuous in blue mossy aggregates. Under the microscope, 

 large and fairly abundant phenocrysts of anorthoclase lie in an 

 orthophyric ground-mass of sanidine-prisms, embedded in deep-blue 

 pleochroic riebeckite, and a small quantity of interstitial primary 

 quartz. It is this summit-rock which, as already mentioned 

 (p. 305), has been figured by Dr. Harker. 



The augite -olivine -trachyte (PL XLI, fig. 3) is a very 

 hard, compact, grey-green rock, with good felspar-phenocrysts. 

 It weathers to a buff colour, and has an irregular fracture. Its 

 small, fresh ground-mass felspars impart to it a saccharoidal lustre. 

 The phenocrysts are orthoclase, with probably some anorthoclase, 

 and legirine-augite in long thin crvstals showing transverse sections 

 with (100) and (110) developed almost to the exclusion of (010). 

 The extinction-angle of this a?girine-augite is 5° to 9°. The ground- 

 mass in which the phenocrysts lie consists of trachytic sanidine, 

 with fiuidal arrangement, and abundant, fresh, bright-green segirine- 

 augite microlites. The olivine is represented by occasional yellow 

 or reddish-brown pseudomorphs of characteristic shape. 



As the olivine increases, the pyroxene changes from segirine- 

 augite to grey-green augite with an extinction-angle of 43°, and is 

 confined to the ground-mass, which latter becomes orthophyric. 

 Soda-felspars preponderate : both twinned and untwinned albite 

 occur, the latter with a rim of orthoclase. Zircons of large size 

 occur, both in this and in the riebeckite-trachyte. 



This augite-olivine-trachyte closely resembles specimens from 

 Traprain Law and the Bass Rock (East Lothian). It is more 



