﻿584 MK. J. V. ELSDEN ON THE IGNEOUS ROCKS [Aug. 1905, 



almost isotropic substance with pyroxene-cleavages, and sometimes 

 still retaining the usual pleochroism of ferriferous varieties of this 

 mineral. The felspars are mainly of a more acid species than 

 those found in the Llanwnda type. They commonly have a lower 

 refractive index than balsam, are often untwinned, occurring in 

 broad rectangular plates, and a little microcline is sometimes 

 recognizable. The augite does not differ from the normal type. 

 Quartz and apatite are conspicuous in the more acid types. Differ- 

 entiation in situ seems to have taken place to a considerable extent, 

 with corresponding variations in mineralogical characters. The 

 rock is not sufficiently well exposed to establish any progressive 

 change from margins to centre, although probably this is the case. 

 Secondary changes have often obscured the original character of the 

 rock, leading to extensive developments of chlorite, epidote, and 

 zeolitic substances. The rock seems, on the whole, to vary from an 

 enstatite-diorite to an enstatite-diabase. 



As we proceed eastward to Trellys, the character of the rock is 

 seen again to pass into the normal Llanwnda type, which is pre- 

 served through Panteg to Manorowen. 



So far as the Strumble-Head district is concerned, I have confined 

 myself to an endeavour to show that the diabases of Fishguard and 

 Prescelly (although remarkably uniform in character in those areas), 

 when traced westward show local variations, characterized by a 

 tendency to greater acidity and a development of enstatite. These 

 changes, which are most pronounced in Garn Bolch, Garn Fechan, 

 Llech Dafad, and Tresseysilt, are of such a nature as to suggest the 

 intrusion at these localities of a magma of different composition from 

 the typical diabase-magma of the eastern intrusions. 



It will now be necessary to describe the intrusions on the south- 

 west side of the Pwll-Strodyr Fault; and, as it seems to conduce to 

 greater clearness to attack this region from the extreme south- 

 western end, I will next discuss the St. David's-Head rocks, which 

 lie about 12 miles from the above-mentioned fault. 



IV. The Intrusive Bocks oe St. David's Head and the 

 Adjoining District. 



The existence of a quartz-gabbro with rhombic pyroxene at 

 St. David's Head has long been known, but no very detailed account 

 of it has yet been published. Mr. Harker, speaking of a specimen 

 from this district, calls attention to its identity with the rock of 

 Carrock Fell, with the exception that the highly-basic modification 

 found in the latter district had not then been noted in the St. David's- 

 Head rock. 1 The most basic variety in my specimens from this 



1 'Petrology for Students' 1895, p. 6C. 



