﻿Vol. 6 1.] BETWEEN ST. DAVID'S HEAD AND STB.UMBLE HEAD. 589 



The felspars are rather turbid, and the rock is evidently some- 

 what altered, but the analysis corresponds approximately to the 

 following percentage mineral-composition, omitting water : — 



Quartz 1 -740 



Orthoclase 5'560 , « ... ,. v 



Albite 35-6321 f Constituting an oligo- 



Anorthite !"•""' clase-andesineof com- 



55-632 1 J 



7-514/ 1 ,, , 



Enstatite :... .17-028 [ position Ab 2 An v 



Hornblende and augite 16-716 



Ilmenite and magnetite 2*296 



Apatite 1240 



97-726 



In arriving at the foregoing result there is, of course, the usual 

 difficulty with regard to the composition of the pyroxenes and 

 amphiboles, which I have calculated as diopside, with the addition 

 of a small proportion of Tschermak's molecule CaO . A1 2 3 , Si0 2 . 

 As, however, some soda-molecules should probably be included, the 

 albite is, in consequence, too high, and the quartz too low. It is 

 interesting to compare the percentage composition of this rock with 

 some quartz-enstatite rocks of this and other countries : — 



I. II. III. IV. V. VI. 



Silica 54-42 5845 53-50 5071 5403 5672 



Alumina 15'34 1708 22-20 1478 16-71 16-90 



Ferric oxide 0-67 076 3"60 352 1-37 444 



Ferrous oxide 547 4-61 2-64 8-95 770 6-28 



Magnesia 6"69 515 2-00 590 5*66 4-62 



Lime 8-30 7"60 9-45 821 8-84 7'25 



Soda 4-21 4-25 426 276 2-99 465 



Potash 1-01 1-02 0-61 1*39 067 0-63 



Water at 110° Centigr. ... 0-47 0-12 ... ... 0-14 



Water (ignition) 260 0-95 1-50 178 0-53 075 



Titanic acid 072 n.d. 045 192 0"84 



Phosphoric acid 0*57 n.d. ... ... 043 



100-17 9999 100-21 99-92 99-61 10194 



I. Quartz-norite of Penberry Hill. Author's analysis." 

 II. Enstatite-diorite, Penmaenmawr. Analysis by J. A. Phillips, Quart. 

 Journ. Geol. Soc. vol. xxxiii (1877) p. 424. 

 III. Gabbro of Carrock Fell. Analysis by G. Barrow, Quart. Journ. Geol. 



Soc. vol. 1 (1894) p. 323. 

 IV. Enstatite-diabase, Whin Sill. Analysis by J. J. H. Teall, Quart. Journ. 

 Geol. Soc. vol. xl (1884) p. 654. 

 V. Hypersthene-gabbro, Pennsylvania. Analysis by W. F. Hille brand 



U. S. Geol. Surv. Bull. No.' 228 (1904) p. 48. 

 VI. Hypersthene-norite, Oberhofen (Tyrol). Analysis by F. Teller & 

 C. von John, Jabrb. d. k.-k. Geol. Eeichsanst. vol. xxxii (1882) p. 647. 



The close agreement which this Penberry-Hill rock exhibits with 

 that of Penmaenmawr is evident from a comparison of the analyses. 

 This is graphically represented in the accompanying diagram (fig. 1, 

 p. 590) drawn upon Brogger's familiar plan. The Penmaenmawr 

 rock, however, has no hornblende, and a little biotite is occasionally 



