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

Extract

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 goldfields, 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 Palæozoic 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.