G. W. Tf/rrell — Alkaline Igneous Rocks, West Scotland. 79 



and the absence of nepheline. An almost identical rock has been 

 described by Boulton fiorn the Old Red Sandstone between Chepstow 

 and Usk, in Monmouthshire.^ Rocks of monchiquitic habit and 

 composition, but without porphyritic constituents, occur as the 

 contact facies of some kylite and teschenite sills, notably at Craigie 

 and Kilmaurs. 



B. Roeks ivith conspicuotis Nepheline. 



Rocks belonging to this group occur in far less abundance than these 

 characterized by analcite. They comprise theralites and essexites, with 

 an almost ultra-basic type — kylite. The latter, and the essexites also, 

 contain only a small quantity of nepheline, but are treated here because 

 of their relationship to the conspicuouslj' nepheline-bearing rocks. 



1. Theralite. (a) Belloio type. — This rock has been mentioned in 

 connexion with the teschenite-picrite sill of Lugar, in which it forais 

 a stratiform mass, about 10 feet thick, intervening between the upper 

 teschenite and the picrite. In hand-specimens the rock is grey to 

 black in colour, and very fine-grained in texture. Microscopically it 

 is composed of plagioclase, nepheline, titanaugite, olivine, biotite, 

 ilmenite, and apatite in the proportions shown in Table III, column IV 

 (p. 77). Augite is present in two forms — as innumerable minute 

 prismoid grains embedded in the felspathic groundraass, and as larger 

 euhedral pseudo-porphyritic crystals of a deep-purple tint. Olivine 

 occurs in large, fresh, more or less rounded crystals full of magnetite- 

 blackened fissures. Biotite, whilst forming numerous independent 

 flakes, is also commonly associated with very irregular skeletal 

 crystals of ilmenite. All these constituents are embedded in a ground- 

 mass of fresh anhedral felspar and turbid nepheline. The felspar is 

 a highly zonal plagioclase, ranging in composition from Ab]^ An^ to 

 Abg Auj. It is occasionally bordered by a little orthoclase. The 

 nepheline is usually turbid owing to the development of a characteristic 

 streaky micaceous alteration-product, which is arranged parallel to the 

 cleavage. It builds rather large plates to which the felspar is idio- 

 morphic. The felspar and nepheline are crowded with needles of 

 apatite. The rock has a characteristic poikilitic fabric, the granular 

 ferro-magnesian constituents being enclosed in the broad plates of 

 plagioclase and nepheline. . The felspars show systems of radiating 

 fissures springing from enclosed olivine where the alteration of the 

 latter has given rise to serpentine and consequent expansion. 



Another variety (Table III, column V) shows a considerable develop- 

 ment of barkevicitic araphibole, with a diminution of olivine and 

 plagioclase and an increase in the amount of nepheline. I'his rock 

 approaches the Barshaw type described below. 



The rocks described above belong to the theralite group, but have 

 a distinctly melanocratic facies as compared with typical theralites 

 such as that of Duppau, Bohemia. They difPer from the kylite group 

 described below in the small proportion of olivine and the abundance 

 of nepheline. Their textural peculiarities and mineral composition 

 entitle them to rank as a distinct type, which may be named the 

 Bellow type, after the stream in which they are best exposed. Some 

 thin anastomosing veins penetrating the Bellow rock show the same 

 1 Proc. Geol. See, 1911, p. 104. 



