﻿278 ME. J. T. ELSDEX OX THE i^^J IQO^, 



of the types was selected for chemical analysis. I give below the 

 results obtained, reserving for another section of this paper the dis- 

 cussion of the petrographical features in detail. In carrying out the 

 chemical analyses the procedure advocated by Mr. W. F, Hillebrand 

 was followed. The alkalies were determined by the Lawrence- 

 Smith method, which, with proper care, is believed to give very 

 trustworthy results. The combined water, in the presence of a 

 considerable percentage of ferrous iron, could not be accurately 

 determined by ignition ; and either Penfield's method, or, in some 

 cases, the method of direct absorption was accordingly followed. 

 Por the iron-estimations reduction by sulphuretted hydrogen and 

 titration by permanganate was adopted ; and titanium was estimated 

 by the colour-method, with hydrogen-peroxide and standard titanium- 

 solution. 



(a) The Basic Type. 



Specimens of this type collected from Carn Hen, Pen Lledwen, 

 Carn Llidi, Caruedd Gwian, Lloch Cenineu, and elsewhere in this 

 area, show a close agreement in mineralogical character. The rocks 

 consist of a basic plagioclase, rhombic and monoclinic pyroxene, 

 ilmenite, some biotite, and a little apatite. The felspars give fairly- 

 high extinctions measured from the albite twinning-planes, and 

 appear to belong to the labradorite-species. Ehombic pyroxene 

 showing marked pleochroism, but less ferriferous than hypersthene, 

 is about equal in quantity to augite. The latter mineral is brown 

 in colour, often shows strong basal striation, and very frequently is 

 normally twinned. Biotite is present in all the specimens, but in 

 relatively-small quantity. Ilmenite is but moderately abundant. 

 In order of consolidation felspar seems invariably to have preceded 

 the other minerals, penetrating both pyroxenes, and moulding both 

 ilmenite and biotite. 



The chemical composition of this rock was found to be as 

 follows : — 



Molecular 

 Per cent, jproportiona. 



SiO., 49-67 828 



TiO; 1-13 14 



AI2O3 12-46 121 



Fe.Og 1-77 11 



FeO 8-71 121 



MnO 009 1 



CaO 9-57 171 



MgO 10-50 262 



Na^O 2-42 40 



K2O 0-63 6 



PA 013 1 



H,d- 0-37 



h;0+ 2-82 



CO2 trace Specific gravity = 2-96 



Total 100-27 



