﻿594 MR. J. Y. ELSDEN ON THE IGNEOUS ROCKS [Aug. I905, 



the majority of the felspars being more highly refractive and the 

 small quantity of quartz being most probably secondary. The 

 felspars are generally too turbid for determination. Ilraenite, as 

 usual in all the rocks described, continued to separate after the 

 felspars. It is considerably altered to sphene, very little inter- 

 mediate leucoxene remaining. Augite is present, sometimes in rather 

 idiomorphic crystals, but in other cases penetrated by felspar-prisms. 

 That some augite preceded felspar is proved by its occasional in- 

 clusion in the latter mineral. Rhombic pyroxene seems to have 

 been present in considerable quantity, but is now almost entirely 

 altered. Chlorite is abundant, and a little hornblende, associated 

 with augite, may be secondary. Apatite is moderately abundant. 

 There are numerous calcite-areas, and a considerable amount of 

 fibrous secondary mineral resembling tremolite. The rock may 

 be described as a somewhat granular type of enstatite- 

 diabase. 



I have now traced upwards, from St. David's Head to the Pwll- 

 Strodyr Fault, a series of enstatite-bearing rocks which seem to 

 merge into those previously described south of Strumble Head, 

 and afford an explanation of the types of rock occurring at 

 Llech Dafad, Tresseysilt (see PI. XXXIX, fig. 6), Garn Bolch, and 

 the granophyric rock of Garn Fechan. 



Certain rocks of an entirely-different character, which occupy a 

 small area west of the above-mentioned fault, in the Mathry district, 

 remain to be described. 



V. The Lime-Bostonite and Porphyrite-Intrtjsions of the 

 Abercastle-Mathry District. 



I will first call attention to the broad strip of igneous rock ex- 

 tending from Pwll Whiting, through Abercastle to the Pwll-Strodyr 

 Fault. This is coloured in the Geological-Survey map as greenstone 

 on the west of Abercastle, and as intrusive felstone on the east. It 

 is, however, the same throughout, so far as my specimens, and ex- 

 amination in the field, enable me to judge. The rock is clearly 

 intrusive, and alters the shales on the north side in the direction 

 of the dip, that is, on its upper margin. The rock decomposes rather 

 easily, and shows a tendency to present a vesicular appearance from 

 the weathering-out of irregular elongated cavities formerly occupied 

 by a dark material, resembling chloritic matter, which spots the 

 unweathered portion of the rock. A few felspar-phenocrysts and 

 some calcite-areas are visible. The general appearance is that of a 

 greenish-grey trachytic rock. 



Under the microscope, it is seen to consist almost entirely of 

 felspar, chiefly in the form of laths, but some larger crystals are 

 also present. The dark material mentioned above is seen to consist 

 of irregular greenish patches, nearly isotropic, but often associated 

 with a fibrous, feebly-polarizing substance resembling chlorite, and 



