394 F. P. Mennell — Wood's Point Dyke, Victoria, Australia. 



examples show Pericline lamellation as well. The maximum 

 extinction angle in cases where the lamellas give approximately 

 symmetrical extinctions is about 21°, so that the species appears 

 to be andesine. The third variety includes examples which are 

 clear, and are often indistinguishable from quartz, except by 

 convergent light, in cases where there is no twin lamellation. In 

 a few instances they show a decomposed core, and the peculiar form 

 of zoning is also seen which Professor Judd ^ has ascribed to growth 

 after the consolidation of the rock. Well-marked lamellar twinning 

 is sometimes shown, and the species approaches albite, though the 

 outer layers are evidently more acid than the interior, as shown by 

 their increasing angle of extinction. Tliere is yet another form of 

 felspar to be described, that which occurs in considerable abundance 

 intergrown with quartz. In places it gives to a slice the appearance 

 of a typical granophyre. The quartz and felspar occasionally 

 radiate in pseudo-spherulitic fashion from a centre, which is some- 

 times marked by an idiomorphic crystal of decomposed felspar. 

 The felspar of these micrographic intergrowths is usually referred 

 to orthoclase, microcline, or other acid type ; in the present instance 

 decomposition prevents any attempt to determine the variety. The 

 quartz frequently presents beautifully sharp triangular sections, 

 contrasting markedly with the cloudy felspar. Larger isolated 

 granules also occur, but for these a secondary origin is generally 

 to be suspected. 



Of what are usually termed accessories ilmenite is by far the most 

 abundant, being even noticeable on a polished surface of the rock. 

 It is, no doubt, the source of the ' titaniferous ironsand ' so common 

 in the auriferous wash of the neighbourhood. Incipient forms of 

 growth are the rule even in the largest crystals, which are usually 

 built up of plates or rods intersecting at angles of 60° and 120°, the 

 best developed ones giving a somewhat elongated hexagonal section. 

 In many cases the mineral is quite fresh ; frequently, however, 

 leucoxene has been formed through interaction with the decomposing 

 felspars. Occasionally the change has been complete, and nothing 

 remains but a serai-opaque patch of leucoxene, but in most instances 

 the internal structure of the original mineral is revealed in a most 

 striking manner, a lattice-work skeleton of ilmenite being often 

 left which shows up beautifully by reflected light. Apatite is 

 another accessory and is also common. Some of the prisms are 

 short and stout, and show good terminations ; usually, however, the 

 crystals are acicular and very long, several so thin as to be almost 

 opaque, attaining a length of nearly 2 mm. Pyrites and calcite 

 occur as decomposition or infiltration products, but they call for no 

 special notice. There is also a micro- or crypto-crystalline ground- 

 niass present in most slices, and what seems to be some residual 

 glass. It is almost colourless, and looks perfectly homogeneous by 

 ordinary light. Between crossed nicols, however, it shows feeble 

 double refraction, with here and there an approach to spherulitic 

 stiiicture. It has been suggested to me that this might possibly 

 1 Q.J.G.S., May, 1889, p. 179. 



