29 



for the greater part as the variety called chequer albite (pi. i., 

 fig. 2). In composition it approaches a pure soda felspar. 

 The mineral is somewhat kaolinized and spangled with sericitic 

 mica. Th^ presence of this secondary mineral in a soda felspar 

 may be explained in three possible ways : — 



(1) It may be formed from the soda felspar by the intro- 



duction of potash from without, as suggested by 

 F. W. Clarke. (22) 



(2) As most albites contain a small proportion of the 



orthoclase molecule in solid solution the sericite 

 may represent the separation and alteration of this 

 latent orthoclase. 



(3) The sericite may really be a soda mica, and bear the 



same relation to albite as the ordinary potash 

 sericite bears to orthoclase. No data, however, 

 appear to be available as to whether some sericite 

 may be sodic. 

 The chemical characters of the soda aplite are shown by 

 No. 1 of tlie following analyses: — 





I. 



II. 



III. 



IV. 



SiO. .. 



. 76-02 



76-26 



71-18 



78-28 



AU63 .. 



. 14-60 



12-40 



14-89 



12-00 



Fe:03 .. 



0-27 



1-64 



2-11 



— 



FeO 



0-08 



2-60 



1-21 



1-19 



MgO .. 



. 0-04 



tr. 



0-14 



0-37 



CaO 



0-34 



0-28 



0-82 



0-29 



Na..O .. 



. 7-08 



6-27 



6-85 



6-89 



K.6 



0-96 



0-41 



1-70 



tr. 



h;o+ .. 



0-34 



0-12 



0-641 



0-61 



h;o- .. 



0-15 



0-02 



0-24 f 



TiO, .. 



0-07 



0-50 



0-48 



0-34 



Total 99-95 10050 10026 99-97 



Sp. gr. 2-626 2-59 '23) 2*62 



I. Soda aplite, Port Elliot. Anal. W. R. Browne. 

 II. Quartz keratophyre, Mount Remarkable. Anal. E. 

 O. Thiele. Trans. Roy. Soc. S. Austr., vol. xl., 

 1916, p. 583. 



III. Soda aplite, St. David Head, Wales. Anal. A. V. 



Elsden. Q.J.G.S., vol. Ixiv., 1908, p. 284. 



IV. Aplite, James township, Ontario. Anal. N. L. Bowen. 



(Quoted by W. H. Collins in Memoir No. 33, Geol. 

 Surv. of Canada, p. 76.) 



(22) Data of Geochemistry, Bull. U.S. Geol. Surv., No. 616, 

 p. 593. 



(23) Surely thi.s figure is too low for an albitic rock. 



