﻿282 ME. J. V. EISDEX OX THE {^^J I908, 



Calculating from these figures, the following miner alogical 

 composition is obtained : — 



Orthoclase 6-1] 



Albite 29-341 ~n-^■D^ ■ i 



Anorthite 21-40 j =^^''^ Hagioclase. 



Augite 13-92 



Bronzite 18-54 



nmenite and magnetite 6-36 



Total 95-67 



It is evident from the foregoing percentages that the plagioclase 

 is of a more basic type than that occurring in the previously- 

 described rock. There is also a larger proportion of potash, a 

 result which would scarcely be expected. Micropegmatite is rarely 

 exhibited. 



The resemblance between this analysis and that of a specimen 

 of the TThin Sill, described by Dr. Teall,^ is very striking. The 

 two are tabulated together below for comparison : — 



Specimeii fro'/n Whin Sill 

 Cam Llidi. (Bomon Station). 



SiOo 50-55 50-71 



TiO; 1-58 1-92 



Al.Og 15-00 14-78 



Fe.Og 2-54 3-52 



FeO 7-90 8-95 



MnO n. d. 0-31 



CaO 7-85 8-21 



MgO 6-25 5-90 



Jsa.,0 3-53 2-76 



K,0 110 1-39 



h;0- 0-55 1-78 



H,0+ 3-14 — 



Totals 99-99 100-23 



Specific gravity = 2-92 2*94 



There is also a very marked similarity between the petrographical 

 characters of these rocks, although the felspar of the St.David's-Head 

 specimen seems to be rather more basic than the andesine of the 

 "Whin Sill. It is still more striking that the above-named specimen 

 of the Whin-Sill rock contains bronzite, whereas the Cauldron- 

 Snout variety, of slightly-difterent composition, has only the 

 monoclinic variety of pyroxene. The presence of the rhombic 

 variety seems here, as at St. David's Head, to have been mainly 

 determined by the relative proportions of lime and magnesia, as 

 suggested by Prof. J. H. L. Vogt. 



Illustrations of this type are shown in PI. XXIX, figs. 3 & 4, 



1 See Quart. Journ, Geol. See. vol. xl (1884; p. 654. 



