﻿Yol. 6 1.] BETWEEN ST. DAVID's HEAD AND STRUMBLE HEAD. 595 



possibly due to the hydration of an original glass, forming a kind 

 of palagonite, in the manner suggested by Prof. Bonney in expla- 

 nation of a somewhat similar occurrence in the Prescelly lavas. 1 

 Some of these patches have minute spherulites round the margin, 

 showing a distinct cross between crossed nicols. Quartz occurs 

 sparingly, usually in thin streaks and patches, and may be of 

 secondary origin. Except a little granular opaque matter, pro- 

 bably a decomposed titaniferous iron-ore, no dark minerals are 

 seen. Patches of calcite are visible in places, but neither these 

 nor the other secondary material appear to replace pre-existing 

 crystals. (See PI. XL, fig. 1.) 



The felspars are nearly all plagioclase. The laths are rather 

 ragged, and extinguish nearly parallel to their length. The refrac- 

 tive index is lower than quartz. In some cases the symmetrical 

 extinction on each side of the trace of the twinning-plane of albite- 

 twins reaches 10°. Flow-structure is commonly visible, and the 

 felspars are often bent. The porphyritic felspars are apparently 

 more basic than the laths. 



I determined the chemical composition of this rock, selecting the 

 least-weathered specimen obtainable, which, however, was by no 

 means fresh. The following results were obtained : — 



Molecular 

 Percentage. proportions. 



Silica 55-38 -923 



Alumina 18*34 -181 



Ferric oxide 1*13 007 



Ferrous oxide 5"86 "081 



Lime 325 -058 



Magnesia 347 '086 



Potash 0-22 -002 



Soda 7-12 -115 



Titanic acid 0-90 "010 



Phosphoric acid trace 



Carbonic acid 200 



Water (at 110° Centigr.) ....... 048 



Water (ignition) 239 



Total 100-54 



Calculating the albite- and anorthite-percentages from the soda 



and lime, we get 



Albite 60-26 



Anorthite 16 - 12 



•which would together amount to 76-38 of a plagioclase, of composi- 

 tion between Ab 4 An x and Ab^An^ corresponding to an acid 

 oligoclase of low extinction, of which a large portion of this rock 



1 J. Parkinson, ' Igneous Pocks in North Pembrokeshire ' Quart. Journ. 

 <Geol. Soc. vol. liii (1897) p. 468. 



2 t 2 



