Other formats

    TEI XML file   ePub eBook file  

Connect

    mail icontwitter iconBlogspot iconrss icon

Geology of the Provinces of Canterbury and Westland, New Zealand : a report comprising the results of official explorations

The Melaphyre and Quartziferous Porphyry Eruptions

The Melaphyre and Quartziferous Porphyry Eruptions.

Before proceeding to the next, or Waipara formation, it will be useful to treat separately of the intervening period when a series of eruptions, consisting of basic and acidic igneous rocks took place on a large scale along both sides of the Southern Alps, those on the eastern slopes being the most extensive. These eruptions had long ceased before sedimentary beds belonging to the newer era were deposited on their flanks, the material for them being generally derived from their disintegration and destruction. It is impossible to say, at least at the present stage of our knowledge, when the palæozoic rocks described in the previous pages were folded, crushed and denuded to such a remarkable extent, or when they appeared above the sea, and how long they remained above it under powerful atmospheric influences, being subjected at the same time to erosion by rivers or by the sea along the coast, but we can affirm with safety that this period, during which they were denuded to a remarkable extent, was a long one. Moreover, there is sufficient evidence to show that even the forms of several of the present main valleys were already indicated. Thus, when the next period of disturbance occurred, the main configuration of this Island was already so far sketched out that the addition of new rocks made no considerable difference in its outlines. These rocks, as previously observed, consist of two main groups, both of igneous origin, of which the older belongs to the basic and the younger to the acidic sub-division.

page 282

We have no data from which we can judge when those rocks appeared, but there is sufficient evidence to show that they broke through the palæozoic beds upon which they rest unconformably, after the latter had generally assumed their present positions, and their surface had already been denuded and partly decomposed; in fact, a very long period of time must have elapsed before the first eruption took place.