﻿Vol. 6l.] BETWEEN ST. DAVID'S HEAD AND STRUMELE HEAD. 601 



augite, in which, it is often included. In the other groups it is 

 frequently intergrown with augite, and in some cases even encloses 

 the latter mineral. Similar reversals of order with regard to felspar 

 may also be noticed. Sometimes it precedes the latter, and at other 

 times is penetrated ophitically by it. 



Monoclinic pyroxene. — In the diabases augite usually occurs 

 in typical ophitic plates; but granular augite, tending to idiomorphic 

 development, is not wanting in certain types — as, for example, Gam 

 Eawr, Trellys, Llanwnda, and other localities. In the enstatite- 

 bearing rocka idiomorphic forms are, on the whole, more common, 

 and are often polysomatic. Twinning on the usual plan, that is, 

 simple binary twins, the twinning-plaue being parallel to the ortho- 

 pinacoid, is an exceedingly-common feature of the granular augites ; 

 and the so-called sahlite-striation, parallel to the basal plane, 

 with typical herring-bone twins, occurs usually in the norite-group. 

 Pleochroism is not distinguishable, and the colour is generally a very 

 pale brown. 



In many of the rocks there appear to be two distinct generations 

 of augite. In a specimen from Gam Eawr the earlier generation 

 has a higher cy extinction-angle than the later form. The maximum 

 extinction of the most ferriferous variety is about 43°. 



A characteristic feature in all the rock-types is the freshness of 

 the augite. Uralite - alteration is comparatively rare, and the 

 polarization-tints are usually very uniform. 



Hornblende. — The distribution of this mineral is well-defined. 

 In the Carn-Llidi ridge it is wanting or very scarce. In the 

 St. David's-Head ridge it begins to come in towards the east ; but 

 iu the Carnedd-Lleithr Carn-Trelwyd ridge it is prominent, and it 

 occurs also at Penberry Hill, after which it was only noticed at 

 Tnys-y-Castell, where, however, it is present in small amount, and 

 may be secondary. Iu some slides it appears as an outgrowth on 

 augite, and seems generally to be associated with the later stages 

 of consolidation,, being more noticeable in the neighbourhood of 

 quartz-areas. It is pale-brown in colour, with normal pleochroism. 



Biotite. — This mineral has only been noticed in the basic 

 norite-type of the western end of Cam Llidi and St. David's Head. 

 It presents normal characteristics. 



Eels pars. — The felspars generally are not of a very basic kind, 

 even in the diabases ; but the norites appear to contain labradorite 

 of composition Ab ] An 1 . In the diabases the usual type is andesine, 

 but in the quartz-bearing rocks there are frequently at least two 

 generations of felspar, the earlier being the more basic. Orthoclase 

 is apparently present in many of the dioritic types, and is also a 

 constituent of the micropegmatite. Albite characterizes the quartz- 

 albite-enstatite-rock of Porth Gain, and the lime-bostonites have a 



