126 G. W. TijrreU— Alkaline Icpieous Eoclcs, West Scotland. 



Megascopically the lavas are very uniform in appearance. Tliey 

 are all compact, aplianitic, varying in colour from dark red to 

 purplish, rarely grey or hlack. A very characteristic aspect is 

 imparted by a universal red speckling due to the replacement of 

 microporphyritic olivines by red iron-ores derived from the overlying 

 sandstones. A red stain of difPused iron-oxide frequently gives the 

 rocks an appearance of decomposition which is belied by the general 

 freshness apparent in thin section. Porphyritic olivine is invariably 

 present, augite rarely, but felspar never. The above characters 

 easily distinguisb these rocks from the earlier Carboniferous lavas, 

 and have been found very useful in determining the age of some 

 doubtful vents. 



The following minerals have been found in thin section : olivine, 

 almost invariably pseudomorphed b)* haematite, purple titanaugite, 

 plagioclase (Abj^ An-^-Abo Ang), analcite, uepheline, and sparse iron-ores. 



The lavas can be broadly divided into two divisions characterized 

 by the presence or absence of felspar. The felspathic group is the 

 most abundant, and includes femic types of olivine-basalt (cf. Hillhouse 

 and Dalmeny types), analcite-basanite, and analcite-nepheline-basanite. 

 The two latter are probably to be classed with the Kidlaw basalt of 

 the Lothians.^ In the Dalmellington district the vents contain blocks 

 comparable to mugearite and Markle basalt (a non-porphyritic variety). 

 These, however, are extremely rare. The felspar-free rocks include 

 monchiquite, analcite- and nepheline-basalt, and limburgite, types 

 which are also represented in the Lothians - 



The olivine-hamlts consist of abundant micro-porphyritic olivines, 

 pseudomorphed by haematite and serpentine, in a groundmass made 

 up of granular augite and lathy plagioclase, the former usually 

 predominating. The minute interstices are occasionally filled with 

 analcite or with turbid material resulting from its decomposition. 

 Transitional types connect these with the analcite -lasanites, in which 

 analcite occurs in sufficient quantity to rank as an essential constituent. 

 In a rock from the Thornton vent, near Kilmaurs, the analcite occurs 

 as a fresh pellucid, pervading base, enclosing a plexus of augite 

 granules and plagioclase latlis. Olivine and a little augite occur as 

 phenocrysts. In another type from Sorn Hill, near Catrine, the 

 texture is doleritic, and the analcite occupies large polygonal cavities, 

 usually bounded by laths of felspar. It contains detached granules of 

 deep-purple augite, and envelops projecting felspars which appear to 

 have suffered corrosion. Olivine is abundant as phenocrysts with the 

 usual haematite replacement ; also augite in abundant large poly- 

 somatic aggregates of a much paler tint than the augite of the 

 groundmass. This rock also contains nodules of closely packed 

 granules of augite with a little felspar, but no nucleus of analcite. 

 These are comparable with the ocelli found in similar rocks in the 

 Lothians.^ 



A few of these rocks contain nepheline. It is always associated 

 with analcite, and both minerals may occur in a perfectly fresh 



1 Geology of East Lothian, 1910, pp. 106, 111. 

 ^ Ibid., pp. 106-7. 

 ^ Ibid., p. 109. 



