﻿Vol. 66.] DYKE AT CROOKDENE (NORTHUMBERLAND). 



Basalt (fresh). Basalt (weathered) 



Si0 2 53-51 52-18 



Alo0 3 15-18 2245 



TiOo 104 1-31 



Fe.,0 3 10-47 9-55 



CaO 1017 989 



MgO 7-14 1-19 



KoO 062 076 



Na 2 1-87 2-67 



Totals ... 100-00 100-00 



The figures in the second column, representing the composition 

 of the unaltered portion of the weathered rock, indicate that this is 

 essentially a felspar allied to labradorite. Eeplacement of the 

 augite by calcium carbonate has left the remainder of the rock 

 poorer in magnesia and correspondingly richer in alumina, with- 

 out seriously affecting the other constituents, whence it may be 

 inferred that the augite is rich in magnesia and has a silica-content 

 approaching that of the rock itself. 



The Felspar Aggregates. 



To prepare the felspar for analysis it was crushed so as to pass 

 through a 30-mesh sieve, but to be stopped by one with 40 meshes 

 to the inch ; then digested with cold, very dilute hydrochloric acid, 

 which took out some iron, calcium, magnesium, and aluminium 

 present as readily soluble sulphates, carbonates, and silicates. After 

 washing and drying, the powdered mineral was brought into a 

 solution of boro-tungstic acid of specific gravity 2'69. This caused 

 a good separation, the weathered mineral (sp. gr. = 2*671) rising 

 to the surface and the fresher portions sinking. Further purification 

 was effected by hand-picking. 



The analysis of this mineral is tabulated below (I). Under II 

 are the figures for the aggregates of the Collywell Dyke, and under 

 III the analysis of the porphyritic felspar of the Tynemouth Dyke, 

 quoted from Dr. Teall's paper (Quart. Journ. Geol. Soc. vol. xl, 

 1884, p. 234). 



I. II. III. 



SiOo 45-88 46-61 47"30 



A1 2 3 34-31 3513 31-50 



Ti0 2 004 013 — 



Fe o 3 083 0'25 1'85 



CaO 18-28 16-74 14-88 



MgO none none 093 



K 2 011 015 038 



Na 2 0-82 105 122 



H 2 014 0-22 1-80 



Totals... 100-41 100-28 99-86 



Specific gravity ... 2'703 2-729 



1. Felspar aggregates .of the Crookdene Dyke. II. Felspar aggregates of the 

 Collywell Dyke. III. Porphyritic felspar of the Tynemouth Dyke (analysis 

 by Stead). 



