BY W. H. BRYAN, B.SC. 151 



Intrusions in ihe Granite. — The majority of the 

 dykes found penetrating the granitic rocks can be 

 divided into two distinct classes, viz., dark porphyr- 

 itic rocks, and light coloured aplites. Seve;al ex- 

 amples of the former type can be seen in the cuttings 

 along the Waterworks Road. They vary in width from 

 a few inches to about fifteen feet. A fairly typical example 

 of this class occurs in the granite quarry off the Waterworks 

 Road. This, is about nine inches wide, is dense, heavy, 

 grey in colour, and made up of phenocrysts of a light 

 brown (altered) plagioclase and small patches of secondary 

 quartz set in a very fine-grained felsitic groundmass. 

 Scattered throughout the rock are segregations of pyrites 

 up to 5 m.m. in diameter. The other dykes of this type 

 differ chiefly in the proportion of the phenocrysts to the 

 groundmass, this latio being very variable. These 

 intrusions are similar in their general characteristics to those 

 considerably larger tlykes outside the granite to be 

 described later as the '' Porphyries." 



The aplitic dykes are considerably more numerous 

 than the porphyritic. They generally occur somewhat 

 near the contact of the granite with the schist, where they 

 are frequently associated with pegmatites. These latter vary 

 from rather coarse-grained rocks — with crystals of ortho- 

 clase, about two inches long — to micropegmatites, Avhose 

 typical structures can only be observed with the micro- 

 scope, and are often found near, and probably connected 

 with, large masses of quartz. The aplites themselves are, 

 as a rule, very fine-grained. They vary considei-ably in 

 colour — different examples being white, light brown, pink 

 and red — and are generally almost entirely free from 

 ferro-magnesian minerals. A rather different type of 

 rock, but one which probably belongs to the same phase, 

 is also met with. This occurs well within the granite, 

 in large irregular masses. It is a little coarser grained 

 than the typical aplites, and is generally red in colour. It 

 frequently contains small blebs of quartz as phenociysts 

 and abundant black tourmaline arranged in stellate groups 

 of small acicular crystals. In this and other points this 

 rock seems to resemble rather closely the aplitic dykes 

 intrusive into the Acid Granite of New England, a& 



