J. A. Thomson. — Igneous Intrusions of Mount Tapuaenuka . 311 



pilite, in accordance with current practice, although the original pilite of 

 Becke was a mixture of chlorite and tremolite after olivine. Another mode 

 of alteration is into iddingsite, which can be easily distinguished from the 

 biotite present m the rocks by its greener colours, poorer pleochroism and 

 cleavage, and less-regular birefriugence. It frequently retains the schiller 

 structure of the olivine. Serpentinous alteration has not been observed in 

 any of the rocks. 



The augite is a pale lilac-brown titaniferous variety, and frequently 

 exhibits zoning characterized by varying intensity of colour. It has a 

 dispersion so strong that many sections do not yield complete extinction. 

 The axial angle is moderate, the mineral being optically positive. The 

 commonest alteration is to a bright-green uralite both around the edges 

 and along shear- zones (see fig. 2). In many of the rocks the former 

 presence of augite is inferred from the inclusion of greenish or colourless 

 amphibole withia brown hornblende. 



Amphiboles of various colours and habits are present in most of the 

 rocks. Original common brown hornblende is very common. In the basic 

 dolerites it occurs in subordinate quantity, mostly on the exteriors of augite 

 crystals, and is generally in parallel crystallographic position to them. 

 Sometimes it is intergrown marginally with the augite ; at other times it 

 appears to result from a partial magmatic resorption of that mineral. In 

 the rocks with lamprophyric afiinities it forms independent crystals or 

 encloses only relatively small cores of augite, and in the spessartites forms 

 also elongate prisms of a second generation. In the acid rocks the original 

 hornblende is a green variety with brown tones. Green hornblende, more 

 or less fibrous, is common as uralite. In many cases the original crystals 

 of brown hornblende contain outgrowths of massive green hornblende 

 which must be interpreted as migrated uralitic material from the augites 

 undergoing that change. As before mentioned, tremolite is found along 

 with talc in pseudomorphs after olivine. 



A strongly pleochroic biotite is common in most of the rocks, but is some- 

 times absent. In the dolerites it is usually subordinate to the olivine and 

 augite, and is clustered around these minerals and the iron-ores. Very 

 beautiful intergrowths with augite occur, and in one case a triple intergrowth 

 of augite, biotite, and brown hornblende was observed. The relative 

 abundance of biotite in the basic rocks points to a higher potash-percentage 

 than in normal dolerites. 



The iroyi-ores appear to belong to magnetite in all the rock types, and 

 may often be seen by their decomposition-products to be titaniferous. In 

 the doleritic rocks they form large allotriomorphic masses, being moulded 

 on the other minerals ; they are seldom decomposed, and show no traces 

 of the rhombohedral structure of ilmenite in reflected light. In the lampro- 

 phyric rocks they form small idiomorphic octohedra, and are often altered 

 in part to a white leucoxenic product. Alteration to limonite is also not 

 uncommon. Pyrite is very abundant in all the rocks, and is generallv 

 present as a replacement of the magnetite. 



The feldspars consist of plagioclase in the basic and subbasic rocks, and 

 of potash-feldspars and plagioclase in the acid rocks. The species of plagici- 

 clase ranges from bytownite in the most basic dolerites to albite in some 

 of the rocks with lamprophyric affinities. The basic feldspars are generallv 

 clear, but the more acid plagioclase and potash-feldspar are extensivelv 

 sericitized, so that identification is sometimes impossible. In the granites 

 a microperthite forms the dominant feldspar, but owing to its state of 

 alteration the nature of its components has not been deternained. 



