SCIENTIFIC TALKS.

Otago Witness, Issue 1989, 10 April 1890, Page 43

SCIENTIFIC TALKS.

BY F. G. S.

VOLCANOES.

X. Before abandoning this subject, whioh has already, perhaps, claimed more than a fair share of attention, but whioh we in New Zealand must always regard with peculiar interest, it may be useful to consider from a geological point of view, the influence of a rolcanic eruption on the district which has been unfortunate enough to experience ifc. The effect produced by showers of dust and ash are constantly brought before us by the beautiful photographs of the Tikitapu bush, before and aftei the Tarawera outburst, which are exhibited by our photographers. From these pictures we learn the desolation and destruction brought by a deluge of this nature. Nor are any of us who were in the colony at that time likely to forget the thrilling accounts of death and misery which' wore then current. But there are other details to be considered in the light of subsequent history. Time heals all things, and none more rapidly than wounds on the face of our mother earth.

In considering the geological action of water we noted how the hardest rocks are gradually worn down, their sharp edges rounded off and their asperities removed, and how enormous quantities of heavy material are carried along on the bosoms of the rivers. The genial rain soon spreads a mantle of verdure over the most desolate landscape, and the soft carpet of moss and grass rapidly causes any but a perfectly sterile soil to assume almost all the beauty of a cultivated pasture. In some cases the ground appears to be hopelessly barren, but in the case of Tarawera, which we may appropriately conBider first, this is fortunately not the case. Afc the time of the unhappy occurrence grass was buried, bush was broken down, and smothered in mud caused by the mixture of the volcanic ash with copious rain, and the settlers saw with dispair nothing but ruin staring them in the face.

It is interesting to note the different effects of a covering of volcanic ash on various classes of vegetation. For instance, while the large hard-wooded trees in the Tikitapu bush were killed, the small, soft wooded varieties put forth, during the following summer, numerous fresh shoots. Within five weeks of being covered with 4£in of mu3, the docks at Ngae were showing above the ground, although it was midwinter; and the plant which appeared able to penetrate the greatest thickness was noticed afterwards to be the bracken fern which is so common. A layer of ash 2ft in thickness was unable to suppress this hardy plant.

Now the Tarawera oountry was originally covered with a soil derived from rhyolitic or acidic rocks, containing on an average about 74 per cent, of silica and only 26 per cent, of bases which comprise the useful ingredients for the nourishment of plants. Thus the soil was too light, more especially as much of the silica was in the form of quartz crystals, which- merely resist decay and render the soil still lighter. . The ash which fell consisted of basic ingredients which, when they decay, furnish a large amount of material which is useful as plant food. This was already apparent in the summer of 1886-87, when the farmers declared that never before had they experienced so abundant a supply of grass. In the swampy lands, too, where the ground was very low-lying and rich in vegetable matter, the addition of a layer of ash has raised the level of the soil without at all deteriorating its quality. As we have recently read in a local paper, the neighbourhood of Krakatao, which was left bare and desolate by the eruption of 1883, has for a long time begun to cast off its barren aspect, and a beauteous cover of vegetation has abeady to some extent begun to cover up the dust and ashes which formed a fit garb of mourning for that awful catastrophe. On the physiographical features of a district the effect of a volcanic eruption is often very striking. We have already seen how cities and the remains of human beings, with coins and ornaments, pottery and works of art, may be buried for long ages beneath a. protecting layer; but before the dawn of man's existence on the earth, organic remains were covered and stored up in a similar manner ; and from these vre can now determine the geological ages of the deposits, as registered in unmistakeable language by a hand which never errs, and written with the pen of Nature, who knows no errors or mistakes.

So much for the effects of showers of ashes, but we must not forget the lava which issues from volcanic vents, and forms a solid crust over the face of the country. This is naturally more violent in its action though less far-reaching in its influence, than the ash and mud. Sometimes a valley which has beer in long process of excavation by the gentle atmospheric influences, or which was in ages past ploughed out by the slow but irresistible agency of ice, may be filled up ia a single day ; or the molten rock may form a bar across a stream, and make a sheet of water, such as exists at the picturesque Lake Aid at, in Auvergne. There are two very different ways in which an outpouring of lava may affect a district, In the first place, the whole country may be oovered under a dense sheet of rook; rivers, lakes, ravines, and valleys, all obliterated by the solid crust of lava ; in this way the great basalt plain of the Snake river, in Idaho, has been formed ; and here in Otago we have many

examples of this kind of deposit. When a locality has suffered in this way the whole character of the country is changed, and the various operations of running water are compelled to commence again in new courses. In the second place the hot lava acts directly on the rocks, fusing and altering them, and causing what is known as metamorphism. Nothing shows this alteration in a more striking manner than a seam of coal, and in parts of New Zealand we may see brown coals whose volatile constituents have been driven off by the proximity of heated rocks, and whioh have become in consequence anthracitic in oharacter. In other cases the heat causes large deposits of snow to be suddently melted and produces in this way disastrous floods.

So far we have discussed only the damage and desolation caused by volcanic action, with incidental mention of the improvement of the soil in one instance; now let us see what industrial products are derived from these great laboratories of Nature. In the first place, nearly all the sulphur of commerce is derived from volcanic districts, and White Island, which belongs to the belt including Tarawera, has furnished large supplies of this useful material. Some of the lavas form good building stones and road metal, for example of which we need go no further than the "blue-stone" of our immediate neighbourhood. The stone of which the Port Chalmers dock is constructed is a volcanic breccia, which seems to require careful selection, as while some of it [makes durable building and curbstones, other portions appear entirely unable to withstand the ordinary atmospheric agencies. Tougher lavas make milling stones, and some of the finer varieties are cut and polished like marble. Pumice is too familiar to need a detailed notice, and obsidian or volcanic glass, large quantities of which may be seen in the Exhibition, was used by the ancients for mirrors, and by savages for knives, arrow heads, &c. In the cavities of [the jolder lavas are found most oE the agates, jaspers, chalcedonies and carnelians, so favourably known as " Scotch pebbles." Besides sulphur, sal ammoniac and borax are volcanic products, and at the Cape of Good Hope, diamonds are derived from necks or pipes which are supposed to be ancient volcanic vents. The soils too which are derived from the disintegration of many basalts are extremely rich, and the fertile character of much of the land immediately surrounding Dunedin, Oamaru, and Timaru is due to this material. In the next article we shall deal with the place occupied by vulcanism in the great scheme of Nature.