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. 
Orthoclase may be absent. Such aplites are therefore often 
Examples of this type have been described by Elsden, 
Bowen,09 Collins,@6 and others. 
To be correlated here also are the albite— rich dyke foc 
described by Turner, Duparc and Pearce, and Ransome. 
The former are associated, as has been noted, with serpentine 
and gabbro masses respectively. The albite rocks described by 
Ransome are associated with diorite. 
e micropegmatite of the Purcell Sills,“ 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. 
efore discussing the mechanism of the differentiation 2 
the Willoughby aplites and pegmatites, the characteristics 0 
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 ; 
e microcline aplites consist essentially of fine-grained 
of quartz, biotite, and more rarely | acid plagioclase m 
present. Granophyric phenocrysts of microcline and quartz 
so occur. 
The albite pegmatites (albitites) are composed essentially 
` of albite with quartz (quartz albitite), of dominant albite Asi 
accessory muscovite (muscovite albitite), and are comparatively 
coarse grained, sufficiently so to texturally determine them as 
. Pegmatites. Microcline appears to be absent. 
09J. V. Elsden: Q.J.G.S., 1908, vol. 64, p. 273. 
09 N. L. Bowen: Journ. Geol, 1910, vol. 18, p. 658 
; 49 W. H: Collins: Mem. 33, Geol. Surv. Can., 1913, p. We 
191 52 F. L. Ransome: Journ. Wash. Acad. Sei., vol. i, No. 4 
1911, pp. 114118. . 
, (9S. J. Schofield: Mus, Bull. 2, Geol. Surv. Can., 1914, 
709 F. F. Grout: Econ. Geol., vol. 13, No. 3, 1918, p. 185. 
