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



III. — The Wood's Point Dyke, Yictoeia, Australia. 

 By F. P. Mennell, F.G.S., Curator of the Ehodesian Museum, Bulawayo. 



THE Wood's Point Dyke is intrusive in a belt of Silurian (Upper 

 Silurian) strata which strike in a direction somewhat west of 

 north and extend beyond Walhalla on the south. Wood's Point is 

 about 75 miles from Melbourne in an easterly direction, and is not 

 situated on the coast as might be inferred from its name. The 

 nearest sea-water is, indeed, some 60 miles distant. The dyke has 

 a north-west by south-east bearing, and it may be taken as typical 

 in many respects of the intrusions which are frequently associated 

 with the Silurian rocks of the Victorian goklfields, though it presents 

 several peculiar features of considerable interest. It penetrates strata 

 of different lithological characters, and, as might be expected, varies 

 considerably both in texture and in the relative abundance of its 

 mineral constituents along its course. That absorption of the 

 surrounding rocks has taken place to a considerable extent may 

 be taken for granted from certain of the modifications which are 

 exhibited. In several particulars, however, specimens from different 

 points along the outcrop agree, not only with one another, but with 

 many of the other Victorian Palasozoic but post-Silurian intrusions. 

 We have, indeed, an illustration of what seems to be a general rule, 

 that precisely similar minerals tend to develop in all parts of the 

 same magma, even when the various portions differ considerably in 

 chemical composition, unless the variations reach an extreme point. 

 In other words, the relative acidity or basicity of different portions 

 are expressed by differences in the proportions of the same minerals 

 rather than by the development of other minerals of more acid or 

 more basic character. 



Hand-specimens representing the average appearance of the 

 Wood's Point rock ai-e dark-coloured and distinctly granitic in 

 texture. Tlie specific gravity ranges as high as 2'9, and the 

 closeness of grain enables it to retain a good polish which might 

 make it valuable in the future for ornamental purposes. Under 

 the microscope it is seen to depart widely from a truly plutonic 

 structure. Hornblende is the dominant constituent, though both 

 augite and enstatite are also present. It forms larger crystals than 

 any of the other minerals, but it is certainly not ' porphyritic ' in 

 the strict sense of the term. The word ' porphyritic,' if it has 

 any real scientific application at all, must be held to characterize 

 a mineral as of prior consolidation to, if not as actually belonging 

 to an earlier generation than, the remaining constituents. In the 

 present instance, despite its relative development, the hornblende 

 rarely shows the crystal outline which would indicate early con- 

 .solidation. In fact, it is sometimes moulded on felspar, and there are 

 even indications of an ophitic structure in the occasional penetration 

 of a crystal by idiomorphic crystals of the latter mineral. The 

 colour of the hornblende is brown, which, as pointed out by Teall,^ 

 is seldom or never the case with the varieties derived from augite 



1 "British Petrography," p. 166. 



