Art. XLV.—Pebbles and Drifting Sand.

By E. W. Andrews.

[Read before the Hawke's Bay Philosophical Institute, 11th September,1893.]

It has long been a proverb that “the constant dripping of water will wear away a stone.” The specimens that accompany this short paper illustrate the central truth of the proverb in the case of another agent, for they show how drifting sand wears away stones.

The first set of specimens is from Wanganui. Just below the bluff on the Wanganui River, towards the South Spit, is a series of low dunes across which the sand is constantly blowing. In the centre of these dunes is a small level space, composed of clay, in which are embedded pumice-blocks, pieces of driftwood, and other débris, probably brought there periodically when the river is in flood, or when the sea washes over the sandhills at unusually high tides. For some reason or other the sand never stays on this clay-bed, but sweeps across it from dune to dune; and, as it sweeps across, it polishes quite flat all the débris, fixed in the clay. I show herewith two pumice-blocks and a piece of wood thus worn away.

The other set of specimens comes from Wellington. Between Island Bay and Evans Bay—that is, between the harbour and the open sea—there is a narrow tract of land shut off by hills on the east and west, but open on the north and south. The result is that the wind that blows across this narrow isthmus has mainly either a northerly or a southerly direction, and the drifting sand, instead of polishing the stones flat (as it does at Wanganui, where the wind can come from any point of the compass) polishes them at a double slope, and gives them the shape of a Brazil nut. Sometimes, by some chance, these stones get shifted, so that their new axis is at right-angles to their old one, and then they assume a conical form. This form is necessarily the least often met with, and I have only one rather poor specimen, though I have seen much better ones.

I would be interesting if those who see these stones this evening would, when crossing a sandy tract, keep their eyes open for any pebbles that illustrate the erosive action of drifting sand. Facts of this nature, though small in themselves, often enable the geologist to formulate a new theory. For instance, the stones that are before you to-night might form a text for a sermon on the shaping of mountain-ranges by atmospheric dust—a sermon which I am sure you will be delighted to hear I am not now going to preach.