﻿Yol. 64.] ST. DAYID's-HEAD ^ ROCK-SEEIES.' 279 



Calculating the mineralogical composition of this rock from the 

 molecular proportions, the following approximate result is obtained, 

 omitting water : — 



Inorthite-::;:;:::::::;:::::::::: To} ='^-^ L^b-aonte. 



Bronzite 22*5 



Augite 22-2 



Biotite 6-5 



Ilmenite and magnetite 4"1 



Apatite 0*3 



Total 96;5 



That this is a thoroughly-basic rock is evident. Although no 

 olivine was recognized in anj^ of the specimens, the invariable 

 presence of biotite in this type and its absence from all other types 

 is significant, and seems to suggest that at a certain concentration 

 the orthosilicate- molecule may, in combination with potash and 

 alumina, form biotite in place of olivine. 



This rock is interesting, as being of a type not very common in 

 the British Isles, where distinctly-basic rocks of the gabbro-class, 

 with prominent rhombic pyroxene, have not often been noted. The 

 analysis shows considerable resemblance to certain rocks from the 

 United States and elsewhere, described as norites, analyses of some 

 of which are tabulated below : — 



T. II. III. 



SiO^ 49-67 48-23 49-28 



TiO^ 1-13 1-00 0-87 



AUO3 12-46 18-26 15-76 



Fe;03 1-77 1-26 1-86 



FeO: 8-71 610 8-94 



MnO 0-09 ... 0-20 



CaO 9-57 9-39 10-51 



MgO 10-50 10-84 8-21 , 



Na^O 2-42 1-34 2-58 



K,0 63 0-73 0-76 



P;0, 0-13 0-07 0-11 



h;0- 0-37 0-26 0-47 



H;0+ 2-82 200 1-10 



CO2 trace 0-43 036 



Totals 1 00-27 9991 99-0 1 



I. Biotite-norite, St. David's Head (Pembrokeshire). Analysis by A. V. 

 Elsden. 

 II. Bronzite-norite, Crystal Falls (Michigan). Analysis by G-. Steiger, 

 Journ. Geol. Chicago, vol. vi (1898) p. 382. 

 III. Horublende-norite, Prospect Hill, Litchfield (Conn.). Analysis by 

 W. F. Hillebrand, Bull. U.S. G-eol. Surv. No. 228 (1904) p. 43. 



In my former paper I called this rock a ' biotite-norite,' and, 

 in spite of the vague signification which the term norite has 

 acquired, it seems the most convenient name to adopt in this case. 

 In both chemical and mineral constitution this rock differs essen- 

 tially from the normal gabbros and diabases. The analysis shows 



