322 Tra nsa c t io us. — Geo Jogy . 



amidst the lava, a curious cast of a piece of charcoal froui a 

 stick about 2^in. in diameter. 



The degree of decomposition of the gravels into soil and 

 subsoil, before the lavas overspread them, is proof of long 

 exposure to atmospheric and organic influences. I doubt 

 whether any portion of the grey gi-avels of the plains has been 

 decomposed into soil and subsoil to a greater depth or more 

 thoroughly. Supposing the degree of decomposition to be 

 equal, and to have been effected under similar conditions, then, 

 whatever age may be assigned to the present plains now, an 

 equal age must be assigned to the red-gravel surface at the 

 time of the volcanic eruption. And tins age is not the full 

 measvu-e of the lapse of time between the two cold periods. 

 The dolerite is now covered with loess, which I believe to be 

 one of the earliest products of the second cold age, and the 

 lava was certainly exposed to atmospheric denudation for a 

 long time before any loess was deposited upon it. If we may 

 judge by the roughness of surface of the lower of two streams 

 worked in one of the quarries, the surface of the fresh lava- 

 streams was covered with minor rugosities. The doleritf 

 appears to weather with extreme slowness, yet it was exposed 

 after eruption, and before any loess fell upon it, long enough 

 to permit the whole surface to be smoothed down, and the 

 edges of the rude prisms into which the rock split on cooling 

 to be rounded off, and the cracks in most cases to be widened 

 into little gutters. There has been practically no alteration of 

 the dolerite since. The loess goes hard down upon the rock, 

 or a fine brown mud fills all crevices and forms a level-topped 

 layer over it. It is only in small hollows or pits in the rock, 

 which would early be filled with rain-wash, that the rugged 

 siu'face of the fresh flow has been at all preserved. 



A proof of the great absolute age of the red gravels is the 

 degi-ee to which they are oxidized from top to bottom. The>' 

 are somewhat less altered beneath the dolerite, I think, yet 

 there they are far more altered than the gravels of the latci- 

 fans. That the dolerite did afford some protection from 

 further alteration appears to be well shown by one interesting 

 fact, which is also an index of the great age of the formation 

 at the time of the eruption. The Timaru dolerite appears 

 everywhere to rest on red gi'avels, except at the south-west 

 corner of Mount Horrible. Near this corner one small ex- 

 posure shows 12ft. or so of loess-like clay, with soil-surface 

 covered by tuff. The clay rests upon rearranged sands, and 

 these on fossiliferous sands, as in the Tengawai terraces. 

 Exactly at the corner, tuff lies upon a few feet of silt and 

 shingle, so loose, so fresh-looking, so unaltered, that the}' 

 could be matched by river deposits only a few years old. This 

 shingle must have been a river-bed at the time of the volcanic 



