﻿284 HE. J. V. ELSDEN OX THE [May 1908, 



or ia perthifcic intergrowth. Analyses of albite-crystals often show 

 a certain percentage both of potash and of lime.^ 



Soda-aplites of somewhat analogous composition have been 

 described by Dr. J. D. Falconer in rocks from the Bathgate and 

 Linlithgow Hills.^ Our specimen is also not unlike Mr. Spurr's 

 tonalite-aplite or yukonite, from the Yukon E,iver.^ Percentage 

 analyses of these and other soda-aplites are tabulated below, for 

 comparison : — 



I. II. III. IV. V. 



SiOo 71-18 71-26 74-79 77-00 74-21 



TiO; 0-48 0-28 01 7 007 0-30 



AI2O3 14-89 11-87 12-59 13-60 14-47 



Fe^Og 2-11 010] ,,Q 0-41 035 



Feb 1-21 2-12/ ^^-^ ... 0-50 



CaO 0-82 2-88 3-58 0-70 1-71 



MgO 0-14 1-08 0-31 0-00 0-28 



Na^O 6-85 6-73 5-10 5-78 7-62 



K,0 1-70 0-05 0-21 1-50 010 



P.p, n. d. 0-10 trace trace 0-07 



H,0- 0-24 0-62 009 023 0-15 



H;0+ 64 2-71 103 048 0-23 



Totals ... 100-26 99-80 99-06 99-77 99-99 



I. Soda-aplite, St. Darid's Head (I'embrokeshire). Analysis by A. V. 

 Elsden. 

 II. Segregation-vein, Kettlestoun Quarry. Described by J. D. Falconer, 



Trans. Koy. Soc. Edin. vol. xlv, pt.^i (1906) p. 147." 

 III. Tonalite-aplite (vukonite). Analysis by H. N. Stokes, Bull. U.S. Geol. 



Surv. No. 228 (1904) p. 270. 

 lY. Aplite (alsbackite), Fallon Hill, Enfield (Mass.). Analysis by G. Steiger, 



ibid. p. 40. 

 V. Soda-grannlite, Mariposa (California). Analysis by W. F. Hillebrand, 

 ibid. p. 240. 



As bearing upon the origin of the aplite-veins of St. David's 

 Head, it is important to note tbat these do not resemble in composi- 

 tion the intersertal portion of the main mass. In addition to the 

 aplite-veins there are also some conspicuous veins of quartzite, 

 often showing great regularity and running in parallel bands in the 

 coarser gabbros. These sometimes reach a foot in width : they 

 must be regarded as secondary, in the present state of our know- 

 ledge. 



I have now shown that in the St. David's-Head intrusions 

 several types of rock, chemically and mineralogically distinct, are 

 represented. These results may be summarized in the following 

 tabular view : — 



^ See Bull. U.S. Geol. Surv. No. 228 (1904) 'Analyses of Eocks from the 

 Laboratory of the U.S. Geol. Surv. 1880 to 1903 ' by F. W. Clarke, pp. 24, 41. 

 - Trans. Rov. Soc. Edin. vol. xlv, pt. i (1906) p. 147. 

 3 BuU. U.S.* Geol. Surv. No. 168 (1900) p. 229. 



