316 DE. r. DixEY ox THE [vol. Ixxviii, 



(4) The Xorite-Aplite. 



Veins of norite-aplite may not infrequently be seen in the norite : 

 generally they do not exceed 1 8 inch in thickness, but in one 

 locality, near York, they are much thicker, and one of them for a 

 short distance attains a thickness of as much as 9 inches. The 

 veins tend to become more acid Avith diminishing width, and so 

 at their terminations they may consist almost Avholly of quartz 

 and micropegmatite. The thin veins, however, Avhich are white, 

 are verv persistent, and can be traced for 20 yards or more across 

 an outcrop of norite "svithout altering appreciably in thickness. 

 The thicker veins consist of a medium-o-rained rock havino" a 

 pale greenish tinge, and made up chiefly of acid soda -lime felspar, 

 orthoclase, quartz, and micropegmatite, with small quantities 

 of pyroxene (both rhombic and monoclinic), hornblende, biotite, 

 and apatite (see Petrographical Xotes, p. 322). 



The norite-aplite- veins are the youngest intrusions of the norite- 

 complex, apart from, tlie dolerite-dykes which are seen to traverse 

 the veins in several localities ; evidence for this may be seen in 

 the Congo Valley, below Hill Cot, where thin vehis of norite-aplite 

 traverse a large intrusion of beerbachite. It is interesting to note, 

 moreover, that the veins are frequently associated with the master- 

 joints of the norite, often running parallel with, or along them, for 

 considerable distances. The affinity of the norite-aplite to the 

 preceding intrusions is indicated, however, in the similarity of 

 these rocks as regards certain chemical and mineralogical characters, 

 such as the presence of rhombic pyroxenes. Xevertheless, there is 

 reason to believe (see p. 317) that the aplite is related to the 

 dolerite as much as, or even more than, to the norite ; also that 

 the aplite bears a magma tic relation to the dolerite similar to 

 that which the segregation-veins and norite-pegmatite do to the 

 norite. If this be so, the norite and dolerite of the complex show 

 an interesting parallel with the olivine-dolerites and quartz- dolerite s 

 of the Edinburgh district, in that these Scottish dolerites are each 

 cut by segregation-veins related in composition to the parent mass.^ 

 Moreover, this view of the magmatic relations of the aplite is 

 supported by the fact that typical gabbro-aplites, such as those 

 occurring in tlie gabbros of Eum and Skye (see below), are free 

 from quartz and micropegmatite. 



The specific gravity of a specimen of norite-aplite taken from 

 one of the thicker veins was found to be 2*72. 



The aplite-veins generally effected a slight alteration of the 

 wall-rocks, shown chiefly in silicification of the felspars and 

 uralitization of the pyroxenes ; this alteration appears in the hand- 

 specimen as a whitish zone, nearly an inch wide, enclosing the finer 

 veins. The margins of the veins are, nevertheless, sharply defined. 



The thin aplite-veins of St. David's ^ differ considerably from 



^ ' The Geology of tlie Xeiglibonrlioocl of Edinburgli ' Mem. Geol. Stirr. 

 1910, pp. 289 & 307. 



2 J. Y. Elsden, ' The St. DavicVs Head Eock Series ' Q. J. G. S. vol. Ixiv 

 (1908) p. 283. 



