﻿Yol. 6l.] BETWEEN ST. DAVJD'"s HEAD AND STRTJMBLE HEAD. 585 



area were taken from the western extremity of Cam Llidi. The 

 dominant rock is noncrystalline, of medium grain, and showing* 

 considerable variation in colour, according to the proportions of 

 felspars and ferro-magnesian minerals present. The specific gravity 

 of the dark variety was ascertained to be 2'98. Under the 

 microscope, the rock is seen to consist mainly of felspar, pyroxene, 

 biotite, and iron-ores. The felspars give extinctions corresponding 

 to a composition varying from Ab 2 An 1 to Ab 1 An 1 , and are of a 

 thoroughly-basic species. Pyroxene is present, in both rhombic and 

 monoclinic forms. The rhombic pyroxene is a fibrous, brownish, 

 bastite-modification of enstatite. It is sometimes of earlier, and 

 at other times of later, consolidation than the augite, with which 

 it is also occasionally intergrown. The augite is brown, granular, 

 and sometimes rather uralitized. It possesses the basal striation 

 so often seen in this class of rock, and twins of the ordinary type, 

 sometimes showing ' herring-bone ' structure, are common. Biotite 

 is moderately abundant, and crystallized after the felspar. The 

 iron-ore is ilmenite, now largely converted into opaque leucoxene 

 and sphene. Quartz is very sparingly present, and seems to be 

 wanting in some varieties. Apatite, also, is apparently absent. 

 The basic character of this rock and the abundance of rhombic 

 pyroxene seem to place it among the norites, using that term as 

 defined by Prof. Zirkel. 1 (See PI. XXXIX, fig. 1.) 



Along the ridge towards the eastern shoulder of Cam Llidi the 

 rock becomes less basic. Quartz-areas begin to appear, and, 

 although my specimens do not here show any micropegmatite, the 

 quartz-areas are invariably associated with rectangular untwimied 

 felspars, having a lower index of refraction than quartz. These 

 appear to be orthoclase. The long laths, on the other hand, which 

 penetrate the pyroxenes, seem to be of a basic kind. The rock 

 exhibits in a marked degree a tendency to the aggregation of 

 enstatite in one portion of the slide and augite in another. A 

 similar peculiarity in hypersthene-rocks has been remarked br- 

 other observers. 2 Both fibrous enstatite and augite occur in large 

 plates, interpenetrated by felspar-laths (see PI. XXXIX, fig. 2). 

 The augite also occurs in more idiomorphic forms, which are 

 nearly always twinned. Biotite gradually disappears with increas- 

 ing acidity, and apatite becomes very plentiful. The norite here 

 passes into a quartz-norite or, as some would describe it, an 

 enstatite-diorite. 



Following the outcrop of this ridge to Carnedd Givian, a similar 

 character is observed. Quartz-areas are associated with broad 

 rectangles of un twinned felspar showing a low refractive index, 



1 Dr.Teall, in describing the Galloway granites, prefers tbe term hy perite for 

 basic, holocrystalline rocks containing both rhombic and monoclinic pyroxene. 

 See ' The Silurian Eocks of Britain ' Mem. Geol. Surv. vol. i (1899) p. (313. 



2 T. H. Holland, ' The Cbarnockite-Series ' Mem. Geol. Surv. India, 

 vol. xxviii (1900) p. 152; also F. H. Hatcb, Quart. Journ. Geol. Soc. vol. xlv 

 (1889) p. 344. 



