﻿280 ME. J. V. ELSDEN OX THE [May 1908, 



a high magnesia-percentage as compared "with both the Carrock- 

 Fell gabbro and the Whin-Sill diabase, both of which are enstatite- 

 bearjng rocks, and the rock is apparently higher in magnesia than 

 the Skye gabbros examined by Mr. Harker. 



An interesting feature also of this rock is, that by reversing the 

 alkalies a typical absarokite-magma would result. 



Illustrations of this type are shown in PL XXIX, figs. 1 & 2. 



{b) The Acid Type. 



The leucocratic variety, as mentioned above (p. 275), is most 



abundant in the St. David's-Head mass, although streaks and 



irregular patches occur in the Carn-Llidi mass. For chemical 

 analysis a sample was selected from near Trwyn Llwyd. The 

 following is its percentage-composition : — 



3folecuIar 



Fer cent. proportions. 



SiO., 52;31 872 



TiO, 1-45 17 



ALA 17-38 169 



Fe;03 2-99 18 



FeO 521 72 



MnO 0-22 3 



CaO 9-95 • 177 



MgO 3-76 94 



Na^O 3-96 64 



K,0 0-75 8 



P2O5 0-20 1 



HoO- 0-30 



H;0 + 205 



CO2 trace 



CI 002 



100-55 Specific gravity = 2-87 



LessO = Cl 01 



Total 100-54 



The theoretical mineralogical composition calculated from these 

 figures is as follows : — 



Quartz 2-1 



Orthoclase 4-4 



^^^^\:; Ill I =60-4 Plagioclaso. 



Anortbite 26-9 J ° 



Augite and hornblende 17' 5 



Bronzite 6'1 



Magnetite and ilmenite 6-6 



Apatite 0'4 



Total 97;5 



This type differs from the basic type of Carn Hen in its greater 

 acidity and distinctly -higher proportion of salic minerals. Free 

 quartz is present, and the potash indicates some orthoclase. Micro- 

 pegmatite is fairly abundant. The felspars belong to rather acid 

 varieties of i)lagioelase, although, when considered together, they 



