426 Transactions. 



of the borders of the crystals to give an appearance approaching that of 

 a mosaic, owing to the rather equidimensional rounded crystals of green 

 amphibole and plagioclase in approximately equal proportions. A little 

 iron-ore is present, and usually sphene, the latter sometimes quite plentiful. 

 Pyrite is a common secondary adjunct. Much of the amphibole is fibrous, 

 and in cases is developed as uralite from central kernels of unchanged augite. 

 In one or two sections the feldspars have a distinct broad lath-like form. 



5. Anorthosite. 



A boulder of this roek about 18 in. in diameter was found in the 

 debris of an outcrop of conglomerate alongside the Albany-Riverhead Road. 

 Macroscopically it is a greyish or bluish-white crystalline rock resembhng 

 marble, and weathering to a white kaolinitic product. It has small patches 

 of green chlorite, and others of a reddish-brown mineral which apparently is 

 a chloritized mica aUied to biotite, and which shows distinct pearly cleavage- 

 faces. 



Seen in section, almost the whole of the rock appears as a highly 

 refractive colourless mineral. Its allotriomorphic equidimensional crystals 

 are very strongly cleaved, are fractured, and are separated by zones of a 

 weakly refractive substance which also penetrates the fractures in the main 

 mineral and has every appearance of being a derivative from it. A Uttle 

 chlorite (perhaps derived from biotite) and zoisite are also present. 



The mineral constituting the mass of the rock shows some perthitic inter- 

 growths, much irregular twin-lamination, and occasional definite twin- 

 lamellae giving extinction angles of 45° on either side* of the composition 

 plane. It was tentatively identified by the writer as a basic plagioclase, and 

 Dr. J. Allan Thomson has been kind enough to confirm this identification. 

 The residuum of feebly refractive mineral was regarded by the writer as a 

 zeolite, but Dr. Thomson suggests that it is largely a more acid feldspar. 



No exact quantitative chemical analysis of the rock was made, but 

 Mr. A. H. Bowell, of Auckland University College, performed tests which 

 showed that it is essentially a silicate rich in alumina and lime, with only 

 traces of iron and potassium, and very little magnesia. No attempt was 

 made to ascertain the sodium content. 



The writer is convinced that this rock is a plu tonic type, for he considers 

 that the finely crystalline material is secondary in origin ; he therefore 

 classes it as an anorthosite, a rock of somewhat rare occurrence. 



6. Dolerites. 



Three specimens of dolerites were collected showing slight differences in 

 the hand-specimen, and only with difficulty separable from the diorites. In 

 mineralogical character they are very similar to the diorites, for they consist 

 of amphibole and basic andesine, the former slightly in excess of the latter. 

 Coarse irregular ilmenite is general, and is sometimes associated with sphene. 

 There is usually a little secondary pyrite and epidote. There are two 

 important differences from the diorites : (a) absence of granulation or other 

 signs of intense pressure, (b) structure. It is on the basis of the structure 

 that they are here classed as dolerites ; the texture is coarse and even- 

 grained, and the amphibole has markedly ophitic relations to the long 

 irregular laths of feldspar, as Plate XXIX, fig. 3, well shows. 



Some of the greenish amphibole is fibrous uralite obviously derived from 

 augite, as in the diorites, for cores of unreplaced earlier mineral survive. 

 Generally the amphibole is a coarse green hornblende, but often it is a finely 

 bladed, or even granular, apparently secondary variety. 



