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PART 1.-THE GEOLOGY AND BOTANY OF THE DUNES. 1. G BOLOQ V . (a.) GENER \b. In contradistinction to the infinite slowness which generally marks the evolution of land-forms, that of those of the dune-area is sufficiently rapid to be witnessed by an observer in a limited lime. A certain definite series of changes can be recognized, leading to a fixed goal, while at the same time retrogression is in progress. Hand-in-hand with the building of dunes under natural conditions, and assisting in the work. goes their occupation by plants, these helping too, in various ways, to hold tin' loose substratum in position, and so keep the hills intact. Such vary from extremely unstable to quite firm structures, on which the erosive power of the wind has no effect. But such stability is at the present time often the exception rather than the rule, certain dune-areas being for the most part made up of bare si retches of sand, protected by a very scanty plant covering, and liable to be displaced by the wind. Man, with his introduced grazing-animals. fires, and methods of cultivation, has further assisted to bring this about, upsetting the equilibrium of the dunes and rendering them still more desert-like. A general knowledge of the changes that dunes undergo, and their causes, is evidently an essential fundamental towards any .scheme for dune-fixing—that is, for modifying the process of nature in a definite manner. (6.J THE MATERIAL OF DUNES, AND ITS ORIGIN. [By li. Speight, M.Sc, F.G.S., Lecturer on Geology, Canterbury College.} (i.) Origin ok Dune-sand. The formation of the sand of dunes commences on our mountain-sides, where, by the disintegrating action of frost, variations in temperature, and other causes, solid rock is broken into fragments, forming screes, and those vaster debris messes locally called "shingle-slips." While these fragments are being carried seawards by the rivers into which they fall, attrition goes on constantly, and additions to the load of the transporting streams are made unceasingly by the country through which it passes, especially in time of flood. Materials of all giades of fineness are thus produced, from boulders of huge dimensions down to almost impalpable powder. Although the term "sand" is usually restricted to those grains varying in diameter between T( i- 0 - in. and J,, in., there is no leal difference as far as mode of formation is concerned between gravel and sand. the one passing into the other by insensible gradations. There is a difference, however, in composition, gravel being usually a collection of minerals cemented and joined together to form a mass of rock, while sand consists in general of the harder and more refractory units of that mass. Sand is also formed by the erosive action of the sea (see Photo No. 25). On every coast, between high- and low-water mark especially, wear-and-tear is constant, material is torn away from its place and reduced in size by the continual friction, as fragments are dashed against cliffs, or are rolled over one another by waves and by strong tides and currents. Sand is thus formed by the sea itself, but the sea also performs an important function in distributing it, however it has been formed. When poured into the sea by streams some of the sand helps to form estuarine and delta deposits and shallow-water marine deposits near river-mouths ; but a great deal is caught by the shore current, added to that produced by marine erosion, and carried along a narrow- belt a little distance from the beach, within which the influence of the waves on the sea-bottom is distinctly felt. While moved along by the shore current, waves and tides carry a part seawards, where it forms permanent marine deposits; they also carry a part landward, where by the aid of prevailing winds it is swept beyond reach of the sea and formed into dunes. This action is more marked during storms, for then the sea-bottom is affected to greater depth, and the shore current is usually stronger, so bhat a greater load can be carried, and additions to the beach, and ultimately to the dunes, are much greater. This is especially the case whim dunes are forming at the head of a bay with gently shelving beach. In fact, shoal water gradually deepening off shore seems essential to the formation of extensive dunes on an exposed coast-line. The amount of sand lying at such a depth that waves can move il readily is. under those circumstances, very great, and vast quantities are gradually brought landward by the waves end ultimately form dunes, especially when the area left by tie' ebbing tide is extensive ami has time to dry before it is covered again by the succeeding tide. When a promontory or obstruction bars the course of the shore current and turns it into deeper water the load is dropped and piled up in front of the obstruction, but usually with an intervening space kept clear by eddying currents. If. bowever, waves and currents are strong they carry the finer particles round the obstruction, and form a beach and its consequent sandhills in its lee. This is occurring in Caroline Bay. near Timaru. The breakwater stops the coarse gravel, but fine sand travels round the end. and is building up a beach, with small dunes on the northern side of the breakwater (see Photo No. 6). If, however, the shore current crosses a shallow bay it forms a spit. This is al first beneath sea-level, but it is gradually built up, and plays its part as a beach, and is finally crowned with dunes. A deep channel is usually maintained close alongside Ihe headland towards which the spit stretches, especially when there is a tidal basin of considerable area, which fills and empties through