332 



as salic interstitial masses or segregations within the associated 

 rock, and are often characterized by a micrographic fabric, or 

 they may occur as distinct dykes cutting the igneous mass. 



The composition of such aplites is variable. The pre- 

 dominant felspar is very often albite or oligoclase, with quartz. 

 Orthoclase may be absent. Such aplites are therefore often 

 characteristically sodic. 



Examples of this type have been described by Elsden,( 14 > 

 Bowen/ 15 ) Collins/ 16 ^ and others. 



To be correlated here also are the albite — rich dyke-rocks 

 described by Turner, Duparc and Pearce, and Ransomed 17 ) 

 The former are associated, as has been noted, with serpentine 

 and gabbro masses respectively. The albite rocks described by 

 Ransome are associated with diorite. 



In the micropegmatite of the Purcell Sills, <18 ) orthoclase 

 is associated with the sodic-plagioclase, and the potassic felspar 

 plays the dominant part in the pegmatites of the Duluth 

 gabbro/ 19 ) 



In all these examples the dominant process of differentia- 

 tion has probably been one of fractional crystallization. 



Before discussing the mechanism of the differentiation of 

 the Willoughby aplites and pegmatites, the characteristics of 

 the types will be shortly reviewed. They may be divided into 

 two groups : — 



(i). Those characterized, by dominant microcline. 



(ii). Those characterized by dominant albite. 



The microcline aplites consist essentially of fine-grained 

 aggregates of quartz, microcline, and subordinate albite. They 

 may pass locally into a porphyritic type in which phenocrysts 

 of quartz, biotite, and more rarely acid plagioclase are 

 present. Granophyric phenocrysts of microcline and quartz 

 also occur. 



The albite pegmatites (albitites) are composed essentially 

 of albite with quartz (quartz albitite), of dominant albite with 

 accessory muscovite (muscovite albitite), and are comparatively 

 coarse grained, sufficiently so to texturally determine them as 

 pegmatites. Microcline appears to be absent. 



(14) J. V. Elsden: Q.J.G.S., 1908, vol. 64, p. 273. 



(15) N. L. Bowen: Journ. Geol., 1910, vol. 18, p. 658. 



(16) W. H. Collins: Mem. 33, Geol. Surv. Can., 1913, p. 59. 



(17) F. L. Ransome: Jonrn. Wash. Acad. Sci., vol. i., No. 4, 

 1911, pp. 114-118. 



(18) S. J. Schofield: Mus. Bull. 2, Geol. Surv. Can., 1914, 

 pp. 1, et seq. 



(19) F. F. Grout: Econ. Geol., vol. 13, No. 3, 1918, p. 185. 



