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The Material composing the Ejected Matter.
Having viewed the deposition of the material, we will now consider its structure and composition.
We have, first, the dry ash laid in the vicinity of Rotomahana (south side), and extending in a gradually reducing thickness to Galatea. Then the mud precipitated over the Wairoa, Rotoiti, Okareka, and Okataina. The dry ash carried in the shape of fine powder over Tauranga, and as coarse sand at Whakatane and Opotiki. Then we have a secondary coating of mud overlying the dry ash in the immediate vicinity of the geysers at Rotomahana, and the varying degrees of fineness of the ash deposited at long distances—notably at Whakatane, where a coarse sand fell for the first few hours, followed by a very fine dust for some hours afterwards. The same circumstance, but in a less conspicuous degree, was noted at Tauranga. In the order as arranged, we find the mud to be chiefly composed of quartz, in the form of fragmentary rock crystal; and as sinter, both white and coloured pink by peroxide of iron; together with a large amount of volcanic scoria in fine fragments, and exceedingly vesicular. This fragmentary scoria we shall find to be in very different proportion as we proceed, and the greatest interest will be felt in this fact, together with its bearing on the future fertility of the soil on which it has fallen, or will itself have replaced. We have not, however, found pumice to any large extent. In some of the older fragmentary rocks isolated patches were attached, but the fine deposits are singularly free from it.*
In addition to these varieties of ash, we have also the solid portions of stone which have fallen, not merely in the vicinity, but also at long distances from the scenes of eruption. The materials thrown out vary considerably. In the immediate neighbourhood of the craters are to be found stones from a few ounces to over a ton in weight.† These vary considerably in
[Footnote] * Some few specimens of newly-formed pumice were afterwards found scattered over the ash-fields, but the quantity is so small as to escape any but the most careful search.
[Footnote] † Some have since been found which would weigh nearly 10 tons.
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formation, but are all portions of the rhyolitic rocks adjacent—a fine-grained tuff and coarse-grained brecciated trachytic rocks being plentiful. In the Wairoa, however, we find both scoria and the cross-grained trachyte just alluded to; while on the eastern end the principal solid material is composed of a basic scoria, in the form of lapilli. Returning now to the examination of the mud and ash, we find that the deposits at Okaro, Wairoa, Tikitapu, and Tauranga are very similar in appearance, being composed very largely of silica, both in the glassy solid crystalline form and as sinter; together with a small but varying proportion of scoria. Coming next to the deposit at Matata and Whakatane, we find the silica in the same forms, but the scoria has increased considerably in proportion. Advancing still further eastward to Opotiki, we find the same characteristics, but the scoria has still further increased in its proportion to the uncombined silica.* Now, if we turn to the analysis we have made of the materials obtained from the places mentioned, we find that they bear out the results of our optical examination. Clearly the ash from Okaro, Wairoa, and Tauranga are of the acidic group, while those from Whakatane and Opotiki are more nearly approaching the basic form. Again, the scoria obtained from Wairoa, and also from the southern end of the eruption, are undoubtedly basic, and have been thrown out in exceedingly large quantities, viewed from the amount and composition of the eastern deposits. Now, hitherto we have had the whole of the rocks of this region placed in the acidic group, and certainly no large mountain masses of a basaltic character could well escape the practised eyes of Von Hochstetter, or the members of the Geological Staff of our Colony. We are therefore forced to the conclusion that large quantities of basaltic scoria were ejected from the Tarawera volcano, or mountain, at the earlier stages of the eruption on the morning of the 10th of June. This is fully borne out by the numerous eye-witnesses, who unanimously speak of columns of fire rushing up from the newly-formed craters, and masses of fire bursting and falling back and around the sides of the mountain. That there was no outflow of molten lava actually discernible after the night in question is accountable by the enormous rush of high-pressure steam carrying off the molten mass in a fine state into the air, where it was carried away by the strong south-west wind which had now commenced to blow, or by being covered up by subsequent deposit of ashes.
We see from the foregoing that we have had two distinct eruptions, the one hydrothermal, the other volcanic, throwing
[Footnote] * The deposit found on the shores of Rotoiti contains large quantities of fine scoria, and as the mountains are approached this increases in quantity and the size of particles, until, on the top of Ruawahia, scarcely anything else is found.
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out differently rocks, acidic and basic, the physical characters of these rocks being as different as their chemical composition. Thrown to a great height, they were caught by the wind-storm and borne along by it in parallel lines from whence they emanated, the acidic to the westward and the basic to the eastward, more or less admixed in the centre, but slightly commingled on the extreme outer edge of the line. In this order they advanced, and in this order were precipitated on the lands over which they passed. Coarse sand, finer particles, dust: thus it was laid, in the order most to be desired by the agriculturist. So fine, indeed, is a large portion of the deposit, that the elements of nutrition in it are available for vegetation almost as soon as the first rains have carried it into the soil; while the particles not so exceedingly fine are already being attacked by that wonderful disintegrator, carbonic acid. For a moment let us glance at the basaltic lava cones in the vicinity of Auckland; and here we find the richest land, capable of growing extensive crops. The more decomposed, the finer the particles, the greater the amount of disintegration: the richer the ground, the greater the profusion of the elements of fertility. And this is the material which has been so lavishly spread over the land on the eastern portion of the district, and which is so largely intermixed with the acidic matter which has fallen over the western. That this rock in its unbroken, undecomposed form, is nearly valueless for plant life we can learn, by turning to the basaltic floes and cinder deposits of Rangitoto; but even there, in the few gullies where rain has washed the dust, and given depth of friable soil for plants to live in, where will we see a richer profusion of vegetation? The result of this downpour over so large an area need not dismay us, but rather give cause for rejoicing that, in the majority of instances, a richer soil has been added than formerly existed; and so lightly and finely has it fallen, that the winter rains will not have passed before it will have been washed into the soil to invigorate the new vegetation and improve the pastures, except in close proximity to the scene of the eruption. Even here we have shown that these deposits are capable of supporting vegetation.