13

C—II

Although the surface of the ground is everywhere composed of most permeable material, the drainage-conditions lead in certain places to the gushing-forth of water in very varying quantities. Tims it may be bo great thai torrents may emerge all ai mice from the apparently solid rock, or quite small trickles spread over the scoria, these latter giving rise to small oases, or, when mure extensive, to bogs, the latter being especially abundant in the neighbourhood of the central saddle. So far as soil is concerned, it may dearly be seen from what has gone before thai there is frequently none in the popular acceptance of the term, the substratum being merely scoria, ashes, pumice, Band, and weathered lava. But in many places, thanks to lapse of time and a plant covering, the above materials have amalgamated, and with the addition of a certain amount of humus a light sandy soil exists for a varying depth, which in certain places is underlain by a sandy clay. Usually there is an miter thin layer of scoria and lapilli : then a deeper layer of sand and humus (frequently very little); then comes the clayey sand mixed with rock fragments. That such a soil is by no means so " | ■ " as might be expected is shown by its plant covering when sufficiently watered. VI. THE PLANT FORMATIONS. (A.) GENERAL REMARKS. It was pointed out in the. botanical survey of Kapiti Island how the plants of any locality are nut arranged by chance, but are found in definite combinations, called technically plant formations, which have come into existence in consequence of the geological history of the region, the climate, the nature, of the soil, and other causes, some physical, others biological. Also it was shown that the. mapping-OUt and describing of such formations 'was one of the first objects of a botanical survey, and that it had important bearings on agriculture and horticulture. In the case of a domain such as the one under consideration these biological entities are also the main factors in determining the scenery, and so their study becomes a further matter of moment when dealing with a scenic reserve. Taking about 3,000 ft. as the lowest altitudinal limit of the park, the vegetation even at this point may be considered subalpine. This perhaps hardly applies either to the large forest-mass on the south and west of Kuapchu, or to those smaller collections of trees occupying the deep gullies on the east of the volcanic plateau, where such mild conditions prevail —especially absence of wind—as to render the formations at this point more of a montane character. The formations in their typical parts are well-enough marked, but between most of them transitions occur. There are really two main classes of vegetation—" forest " and " steppe " —and these, indeed, are oonneoted li\ the subalpine scrub. All the same, large areas of an easily recognised and uniform vegetation occur, each with its peculiar features. Scientifically the formations may be most correctly arranged according to their evolution. Thus the molten lavas, hot ashes, and scoria, burying or destroying all previous plant-life with which they came in contact, would offer an entirely new ami virgin soil for invasion by the plant inhabitants of the unharmed region. Such destruction would be a gradual one, and the new country would, through weathering, transportation of material by rivers, and so on, be subject lo a series of t raiismut at ions which could only be ascertained in part by long and careful research. The general sequence would be —lava and ash-fields; disintegration of lava, and colonisation of ash-fields first and lava at a later date; hollowing-out or reconstructing river-valleys; building up of Hood-plains; cutting of ravines and gullies into these; gradual and frequently quite rapid weathering of these latter through water-action, wind, Ac. : while finally would come the changes occasioned by introduced animals. Thus in such an evolutionary series we should have the solid lava occupied by lichens and small mosses, such as species of Andrecea, followed, as soil crept into the crevices, by an occasional chasmophyte or two. But simultaneously with the lava-flows would the sides of the volcanoes and the lava-flows themselves be covered deeply with scoria and ashes, as has been noted for Tongariro, Ngauruhoe, and Ruapehu at the present time. Thus a soil would be at-once provided without weathering, but which from its extremely porous and unstable nature would be but slowly occupied by those special .verophytes which alone can exist under conditions of excessive drought, fierce insolation, and an unstable substratum. Colonisation of this kind may lie seen at the present time in the case of the new ground formed by the eruption of Tarawera, where, on the mountain-slopes in the very centre of the new ash-fields, within quite a few vears, Raoulia australis has built up its conspicuous silver}' cushions, thanks to its wind-blown seeds. Further developments would occur at a much later date, many incipient plant associations going to the wall with the changes in the topography, as tin rivers formed themselves beds, built up terraces, and flattened out the dibris of the lava and the scoria, pumice, ashes, and sand into plains; finally, melting of the winter snow, gushing-forth of springs, wind, and other causes would seam the apparently even surface with gullies, narrow or wide, deep or shallow, with or without streams on their floors. On these water-built plains the grasses and their accompanying plants could settle down, and within the gullies shrubs and trees, secure from the wind, could form thickets in the first place and finally forest. Hut all I lie above account is of necessity very general. Various features of colonisation must have gone on synchronously. The extreme limits of the ash or pumice deluge would at once receive "heir covering of steppe -grasses, &c, long before the cooling lavas were in a position to foster the nore lowly plants. But, all the same, there would lie a procession of vegetation such as this: Hock-plants, desert, steppe, shrubby steppe, scrub, and finally forest; though, from what has gone before, the rock stage woidd frequently be absent. At the same time, a spring of water would at once offer special conditions in its neighbourhood, and pools with aquatic plants, or bogs varying in their water-content, would be formed without reference to the above detailed procession of events.