﻿586 MR. J. V. ELSDEN ON THE IGNEOUS KOCKS [Aug. I905, 



but there is still ail absence of micropegmatite, and the augite retains 

 a pronounced basal striation. There is also an apparent absence 

 of original hornblende in the slices examined. 



From St. David's Head a ridge extends along the coast, parallel 

 to that of Cam Llidi. Specimens taken at intervals along this 

 ridge show considerable variations in texture. A very coarse- 

 grained variety, recalling in the field the coarse rocks of Llanwnda 

 and Tresseysilt, seems to be capriciously intermixed with varieties 

 of finer texture. All these varieties exhibit more or less micro- 

 pegmatite, which is often very conspicuous. Associated with the 

 micropegmatite and quartz are untwinned felspars, with a lower 

 refractive index than quartz or balsam. Plagioclase with albite- 

 striation is also present, and the extinctions point to a species of 

 andesine. Rhombic pyroxene is abundant, generally in large 

 plates, often intergrown with augite and penetrated by felspar-laths. 

 The relative proportions of rhombic and monoclinic pyroxenes are 

 very variable, one or the other variety preponderating in different 

 slides, or in different portions of the same slide. The rhombic 

 pyroxene is invariably fibrous, and is occasionally brown and 

 slightly pleochroic, but never approaches the character of typical 

 hypersthene. The angite is commonly twinned in simple binary 

 combinations, but more complex twinning is also occasionally 

 seen, the twinning-plane being then oblique to the trace of the 

 vertical cleavage. Basal striation is still conspicuous in some slides. 

 Biotite is absent from all the slices, but a little original, compact, 

 green hornblende may be recognized in some of the specimens from 

 the more easterly portions of the mass. Apatite is present, often 

 in conspicuous quantity, and the iron-ore is invariably ilmenite, 

 sometimes showing good crystalline outlines, with rhombohedral 

 faces, bnt generally altered to leucoxene or sphene. On the whole, 

 the changes noticeable along this ridge in a north-easterly direction 

 seem to indicate a gradual increase in acidity, with the incoming 

 of hornblende in addition to the rhombic and monoclinic pyroxenes. 

 It differs, also, from the Carn-Llidi intrusion in the abundant develop- 

 ment of micropegmatite and an increased proportion of an acid fel- 

 spar, which appears to be orthoclase. The rocks, therefore, would ap- 

 pear to graduate towards the class of ensi:atite-diorites, although 

 there is still a preponderance in places of a rhombic pyroxene. 



East of Cam Llidi, at a distance cf about half a mile, is another 

 ridge, about a mile in length, which rises into the rocky eminences 

 of Carnedd Lleithr, Carn Perfedd, Carn Efald, and Cam Trelwyd. 

 Here the rock is of finer grain, and macroscopically very uniform, 

 looking much like a grey granite. 



At Carnedd Lleithr, a specimen from the south-western corner 

 shows under the microscope a considerable amount of quartz and 

 micropegmatite, the latter often framing the felspar-crystals in 

 the typical manner. The felspars projecting into the quartz-areas 

 all have a lower refractive index than quartz, and are generally 



