Character and Sequence of the Eruptive Rocks.—Melaphyre Series.

Although there are characteristic tufa beds interstratified with the melaphyres, no fossils of any kind have been found in them at present, and thus it is impossible to state when those eruptions, which were on a gigantic scale, took place, by which the melaphyres and their accompanying tufas were deposited upon the sea bottom. I can only repeat what I previously stated concerning them, namely, that they repose on the sedimentary strata, which, previously to the eruption of these basic rocks, had undergone great changes and denudations, and that since then no alteration of importance, except the rising of the land, has taken place. After these basic rocks were deposited, before the next, or acidic beds were formed, they, in their turn, underwent enormous denudations. I may here add, that generally in other countries the acidic formations (quartziferous or felsitic porphyries) constitute the lowest beds, whilst the melaphyres, or basic rocks, repose upon them—our beds thus differing in this respect. Of this denudation the isolated system of the Gawler Downs forms a notable instance, as it is only a remnant of a once continuous series of beds which all around have been removed, often for many miles.

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Sometimes the melaphyres, principally where they occur in higher positions, lie directly upon the edges of the palæozoic rocks, but in most instances, they, are preceded by beds of tufa, which often are So intermixed and blend so thoroughly with the decomposed sedimentary rocks below them, that it is impossible to fix the exact line of contact between them. These lowest beds are mostly wacke, more or less amygdaloidal, of brown and reddish tints, in some localities they are traversed by dykes of melaphyre of a compact texture. They are overlaid by regular coulées or streams of melaphyre. We may consider that some of the wacke and some of the amygdaloids are either ashes, deposited during submarine eruptions on the sea bottom, or if, at least, some of them were formed by regular coulées they have afterwards been so much exposed to hydrothermal action, that they have undergone great alterations and changes in their structure, whilst the melaphyres proper have been ejected during larger submarine eruptions, following each other in quicker succession than the former, by which the whole series became more protected. Some of the streams are of great thickness and extent, as I was enabled to trace them for from five to six miles continuously. Their number is also very great, into which fact the Gawler Downs, amongst other localities, give us a splendid insight. There, streams of melaphyres, many of them standing nearly vertical—a dip of 79° to 82°, generally to the E. S. E., being common-alternate for several miles in an almost endless succession with tufas of various character. Some of them, full of concretions of green earth, are true amygdaloids, or are full of geodes of agate, chalcedony, and all the numerous varieties of quartz usually found in these old basic rocks, vividly reminding me of Oberstein, in Germany.

The lithological character of the rocks under review is very varied. We find some of which the component parts are so distinctly crystallized that they could be mistaken for dolerites, others are more porphyritic, others so compact that they appear like basalts, whilst a few of them have a pitchstone-like appearance. When in an undecomposed state, they are generally black, with greenish, bluish or reddish hues. They contain numerous minerals as accessories, such as agate, chalcedony, amethyst, jasper, calcareous spar, sphærosiderite, delessite, and epidote, but I was unable to find any traces of ores, and the different localities which were pointed out to me as containing copper ores proved unfortunately to consist only of masses of green earth, as accessories to the rocks, or colouring them intensely. Concerning their position, they are invariably found to be below the page 285quartziferous porphyries. In the Malvern Hills they crop up along the sides as well as towards the centre of the acidic zone where the rocks belonging to the latter have been much denuded. In the gorge of the Rakaia, where fine sections are exposed, they occur in the centre, being flanked on both sides by the porphyries. The relations of these rocks to each other are still more clearly exhibited in the Ashburton-Hinds zone, where the melaphyres occupy the outer margin, the quartziferous porphyries forming the centre and covering the former. On Section-plate No. 3 I have copied from my field-books some of the most interesting sections, illustrating the relations in which these eruptive rocks stand to each other.