470 Transactions. — Geology. 



south of the Waitaki, and commencing again near the mouth of 

 the Mataura and continuing, probably, to the mouth of the 

 Waiau, it appears to be connected with the rivers which come 

 down from the glaciers. We may perhaps ask why it is not 

 found also at the mouth of the Clutha. In answer I would say 

 that it has never been looked for in that district. And if it is 

 really absent we should probably find an explanation in the 

 alteration of the courses of some of the rivers in the interior of 

 Otago. 



But, granting that the material came down the rivers, how 

 did it become diffused and spread out over the surface of the 

 land near the embouchures of the rivers ? Evidently it is not 

 accumulating now, for it is covered up near Amberley by the 

 gravels of the Kowhai ; and it is everywhere being washed away 

 by the rain. Also it is clear that it is not a river deposit, for it 

 is too evenly spread, and a river could not possibly have taken 

 it up the hills of Banks Peninsula. Hence we are driven to 

 suppose that it must be either a marine or a wind-borne deposit. 

 It must be one or the other — it cannot be both. 



Sir Julius von Haast was the first advocate of the wind 

 hypothesis.* After reading Richthoven's theory of the origin of 

 the loess in China he came to the conclusion that our silt-deposit 

 was also a loess, and had a similar origin to that of China ; and 

 in this opinion he was supported by Mr. J. Hardcastle.f Rich- 

 thoven's theory is this : He supposes that in the arid interior of 

 a large continent the decomposition of the rocks would produce 

 a great quantity of dust, which would be blown away by the 

 wind. On the outskirts of this arid district a more or less rainy 

 district would be found where grass would grow. The grass 

 would catch the dust, hold it together and grow over it, so that 

 the dust would accumulate and form the deposit called loess. 

 The capillary structure he thought to be due to the roots of the 

 grass which had decayed away. 



In applying this theory to a small island like New Zealand 

 we meet with considerable difficulties. Where are we to find the 

 desert area capable of producing so large an amount of dust? 

 If we suppose that the land formerly extended much farther on 

 each side of the mountains so as to make an arid area, how can 

 we account for the absence of the silt from the interior of Otago 

 and the neighbourhood of Dunedin ( But the silt may have 

 been brought down by the rivers and deposited in their beds, and 

 may have been subsequently blown away as dust-storms over the 

 plains. Here we have a possible source of dust in the old glacier 

 muds, which did not require a desert to form them in. Let us 



*" Geology of Canterbury and Westland," Christchurch, \*~ i -K p. 'M\~. 

 t Trans. N.Z. Inst., vol. xxii.. p, 40fi, and vol. xxiii.. p. 324. 



