406 Transactioyis . — Geology. 



Art. XLVIII. — Origin of the Loess Deposit of tlie Timaru 



Plateau. 



By J. Hardcastle. 



[Read before the Philosophical Institute of Canterbury, 5th September, 



1889.] 



So far as I am aware, three varying opinions have been pub- 

 lished respecting the origin of the clay, silt, or loess which 

 covers the dolerite sheet in the Timaru plateau. 



The late Dr. von Haast ("Geology of Canterbury and 

 Westland,"p.367) adopted Eichthofen's theory, that it is a sub- 

 aerial formation, which has grown up under existing condi- 

 tions, and is still growing. 



Professor Hutton raised some strong objections to that 

 explanation, and ("Trans.," xv., 416) concludes (with special 

 reference to an admittedly similar deposit on the flanks of 

 Banks Peninsula) that " the evidence in favour of the marine 

 origin of this deposit preponderates enormously over the evi- 

 dence in favour of subaerial origin." 



Lastly, Mr. J. Goodall ("Trans.," xix., 457) declares the 

 Timaru loess to be a volcanic ash. 



The object of this paper is to state some evidence which 

 proves that the Timaru loess, and presumably also the similar 

 formation on Banks Peninsula, and others elsewhere, is of 

 subaerial origin ; that it is a formation of wind-borne dust, 

 entrapped by successive generations of dry-land vegetation ; 

 that the whole deposit from base to summit, inch by inch, line 

 byline, film by film, has successively been a dry-land siu'face ; 

 that it accumulated not only slowly, but intermittently, with 

 prolonged periods of pause ; and that its growth was dependent 

 upon a set of climatic conditions which no longer prevail in 

 the neighbourhood. 



For the production of a massive subaerial formation of 

 dust four factors are required — (1) a source of wind-borne 

 dust, (2) winds to transport the dust, (3) vegetation to en- 

 trap it, and (4) suflicient time for its accumulation. In 

 respect of this formation, the first of these factors must be 

 indicated, but need not be located. The second will be 

 granted ; also the fourth. The third will be admitted if it is 

 proved that the deposit was formed on dry land. I leave the 

 indicating of the source of dust for the present, proceeding 

 first to prove that the loess is a dry-land formation. 



The Material. — The material of the deposit is remarkably 

 homogeneous in character, and is almost wholly fine enough to 

 be easily taken up and transported by winds. The excep- 

 tions are a few elements which could not have been wind- 



