Andrews. — On Pebbles and Drifting Sand. 397 



Art. XLV. — Pebbles and Drifting Sa^id. 

 By E. W. Andrews. 



[Bead before the Haivke'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 accom- 

 pany 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 Eiver, 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 drift- 

 wood, and other debris, 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 debris 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. Be- 

 tween Island Bay and Evans Bay — that is, between the 

 harbour and the open sea — there is a narrow tract of laud 

 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 W^anganui, 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. 



It 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 them- 

 selves, often enable the geologist to formulate a new theory. 

 For instance, the stones that are before you to-night micht 

 form a text for a sermon on the shaping of mountain-rano-es 

 by atmospheric dust — a sermon which I am sure you will be 

 delighted to hear I am not now going to preach. 



