Wild. — Geology of the Bluff. 



325 



manner in which the crystals of the principal three constituents are 

 intergrown suggests simultaneous crystallization. The fact that there is 

 no sign of graphic structure, however, and the tendency towards gneissic 

 structure that is observable suggest recrystallization. 



Park,* in 1887, described this mass as an inclusion of the meta- 

 morphic rocks in the norite. But this explanation now seems improbable, 

 as a glance at the following list of analyses will show. A and C are the 

 results obtained above on an analysis of the hornblende schist and the 

 norite respectively. They are repeated here for comparison with B, an 

 analysis of a specimen of the mass under discussion. 



Specific gravity 



2-56 



3035 



2-68 



The analysis shows that the inclusion is more basic even than the 

 norite, so that it cannot be considered merely as an included mass of 

 hornblende schists. For the same reason it is not likely to be due to the 

 complete absorption of a portion of the schist in the norite magma. 



A more probable explanation of the occurrence of this mass depends 

 on the theory of differentiation, to which much importance is attached 

 by many modern geologists. Thus Harkerf says, "We are left free to 

 conjecture that the settling-down of crystals, which seems to be generally 

 ineffective in a sill or laccolite, may give rise to very important differen- 

 tiation in a large intercrustal magma-basin, cooling at an extremely 

 slow rate. Various special features observable in igneous rocks are 

 susceptible of interpretation on this hypothesis, and serve in a measure 

 to support it. The dark basic secretions or ' clots ' wdiich occur sporadic- 

 ally in many granites and other rocks may be taken as an example. 

 These consist in general of the same mineral as the normal rock, but are 

 much enriched in the darker and denser minerals or in those of earlier 

 crystallization. It seems reasonable to regard them as portions picked up 

 from a lower stratum of the magma-reservoir, where crystals of these 

 minerals accumulated by settling down in the magma." 



This theory certainly seems to explain the case in point, where we have 

 an inclusion which, compared with the norite, shows a decrease of 3*7 per 

 cent, of silica, and a total increase of 6 per cent, in the oxides of the 

 banes iron, calcium, and magnesium. Its specific gravity, also, is 3*035, 

 compared with 2' 68 in the case of the norite. 



* Park, J.. "" Notes on the Geology of Bluff Peninsula,'" Rep. N.Z. Geol. Surv.> 



18S7— S8 ry 7*> 



fHarker, A., "The Natural History of Igneous Rocks," p. 322, 190S». 



