152 PROCEEDINGS OF SECTION C. 



higher regions of the earth's crust. The basic portion was the first 

 to ascend, and when it came to rest at some distance below the 

 surface further differentiation separated it into a number of rock 

 types and sub-types differing from each other, but often merging 

 into each other by insensible gradations. 



At a period following closely upon this basic invasion there 

 was an irruption of acidic material which was charged with a high 

 content of aqueous vapour and mineralizing agents The apophyses 

 of the magma penetrate the already solid basic rocks in at least 

 one placed The consolidation products of the earliest phase do 

 not exhibit primary minerals into the composition of which the 

 mineralizing agents enter. 



The magma proceeded to solidify, and the process was marked 

 by the gradual alteration in the composition of the still liquid 

 material through the subtraction of the fractional portion already 

 solidified. Through different fissures, probably due to the con- 

 traction of the cooling solid crust of the magma, the fluid contents 

 were from time to time expelled. 



The tendency for silica, aqueous, boric and other vapours to 

 collect into the heart of the magma was checked in some portions 

 of the mass for reasons thus far not fully determined^, and a 

 number of small separate and isolated segregations of quartz and 

 tourmaline resulted. Thus the nodular tourmaline granite was 

 formed. 



But at least some of these ingredients were gathered together 

 into the heart of the magma. The deeper fissures which tapped this 

 concentration of the more mobile and more acid constituents of 

 the magma allowed the outward passage of the pegmatitic and 

 aplitic material and finally a quartz-tourmaline aggregate. These 

 various dykes which were formed during the various stages of 

 solidification have penetrated not only the granite itself, but in 

 some cases the rocks surrounding the acidic massif^. 



The inequalities of surface produced by erosion have laid bare 

 in different places portions of the acidic and basic masses, the rela- 

 tions of which to each other appear to the author to be in harmony 

 with the sequence which has here been stated. It is true that a 

 large number of chemical analyses must be made before this state- 

 ment of the succession* can be claimed to have a perfectly firm 

 status ; but from the field evidence already collected the writer 

 would submit that it is substantially in accordance with facts. 



1 If the author's contention with regard to the succession is correct, the age relationship between 

 the acidic and basic groups is afforded by this intersection. For the basic rocks are intersected by 

 what appear to the author to be the very earliest consolidation products of the acidic magma — the 

 granite porphyries and quartz-porph\Ties. 



2 The idea at once suggests itself that the nodules might be formed at the periphery of the magma 

 where cooling has proceeded too rapidly to admit of the concentration of the quartz and tourmaline 

 from the many centres at which they had collected. The distribution of the nodular granite with 

 respect to the normal variety has not yet been investigated in detail ; yet, from obser\'ations which 

 have been made, it appears that the nodular variety is not restricted to the outer mantle of the 

 massif. 



3 The neighbouring massif, which outcrops on the coast line between the Reman River and 

 Sandy Cape exhibits similar differentiation phenomena, but not with such variety and detail as 

 does that of Heemskirk. 



4 For example, the referring of the granite porphyry dykes to the very earliest stages of the 

 process of consolidation requires chemical support. The comparison between the composition 

 of these dykes and that of the normal granite is especially required. 



