324 



Transactions. 



In this paper the name " norite ' has been adopted in accordance 

 with the views of Harker and Rosenbusch. 



As has been shown above, the Bluff rock consists essentially of a 

 soda-lime feldspar, a monoclinic pyroxene, and an orthorhombic 

 pyroxene. Both kinds of pyroxene are changing to hornblende, and in 

 the case of crystals, where the change is completed, it is impossible to saj r 

 whether the hornblende is derived from orthorhombic or from monoclinic 

 pyroxene. However, so far as may be judged from what remains, the 

 orthorhombic variety is dominant, and the rock is therefore a norite 

 rather than a gabbro. 



Chemical Composition. — If the chemical composition be appealed to, 

 as some authors demand, it also will be found to support the classification 

 here suggested. 



Si0 2 



Ti0 2 



A1 2 3 



Fe 2 3 



FeO 



MnO 



CaO 



MgO 



K 2 



Na 2 



H 2 and loss on ignition 



48-10 



20-85 

 4-85 



10-55 

 Trace. 

 715 

 3-99 

 0-63 

 2-73 

 I -00 



Total 



Specific gravity. 2-68 



99-85 



2. Basic Secretion. 



At Starling Point the norite encloses a mass of a dark-coloured rather 

 tine-grained rock. Specific gravity — 3'035. 



The size of the mass cannot be made out definitely, as there is a 

 coating of soil at this locality. The outcrop is small, being exposed in 

 a cutting about half a chain in length and 6 ft. or 8 ft. deep. But, 

 judging by the position of the outcrops of norite around it, the surface 

 extent of the mass cannot be more than a chain in diameter. As has 

 already been stated, the norite varies much in texture from point to 

 point, and in some cases approaches to a material similar to that now 

 under description. This led Hutton to describe several varieties of rock 

 from the district. 



Under the microscope the rock presents a similar assemblage of 

 minerals to the norite, but there is a very noteworthy increase in the 

 proportion of ferro-magnesian minerals. Hornblende forms mere than 

 half the rock, magnetite is abundant, there is a little pyroxene, and a 

 basic plagioclase, sometimes containing needles of apatite, forms the 

 rest. There is an approach to a rough gneissic structure. 



Structure and Order of Crystallization. — The rock lias a coarsely 

 schistose or gneissic structure, and this tends to obscure the order of 

 crystallization, so that it cannot be made out with any degree of accuracy. 

 Hornblende seems to dominate, but the edges of its crystals are extremely 

 ragged. In fact, no mineral can be said to be idiomorphic, and the 



