﻿HuTTON. — On the Alluvial Deposits of the Lower Waikato. 333 



GEOLOGY 



Art. LXI. — On the Alluvial Deposits of the Lower Waikato, and the Formation 

 of Islands hy the Pdver. By Captain F. W. Hutton, F.G.S, 



[Read before the AucUancl Institute, 29th 3Iay, 1871.] 



The alluvial deposits of tlie Lower Waikato, between Taupiri and Mercer, 

 consist of reddish-yellow loam, resting on pumice, gravel, and sand. 



The pumice deposit is generally pretty regularly stratified, and can often 

 be divided into three subordinate divisions. The lowest of these is composed 

 of coarse white quartz sand without much pumice, and contains layers of 

 magnetic iron sand. The middle one is pumice gravel, the stones often being 

 of the size of peas or beans ; while the upper deposit is similar to the lowest 

 but with more pumice, and also with pieces of obsidian. The sand is for the 

 most part formed of slightly rolled transparent crystals of quartz. All parts 

 of this formation are sometimes bound together into a hard rock by a ferru- 

 ginous cement. It is not found much below Rangiriri. It is quite evident 

 that nearly the whole of the materials that form this deposit have been brought 

 down from the volcanic region in the centre of the island, and it must there- 

 fore have been formed when the land south of Taupiri was above the level of 

 the sea. The absence of fossils is easily accounted for by supposing it to be an 

 old river bed, for no shells inhabit the centre of the river, and none are found 

 on the present sand-banks ; they only live at the margins of the river where 

 the bottom is more muddy, and where alone vegetation grows. Above the 

 pumice formation lies a fine reddish -yellow loam, with occasionally a few 

 rounded stones of pumice, so much decomposed that they are easily cut through 

 with a spade. I have never found any shells in it, but in places it passes into 

 a whitish sand containing the following species of diatoms, viz. : — Ejnthemia, sp. ; 

 Hir)iantidium pectinale, Kutz; Amphicampa, sp.; SurirellaGraticida,^h.Y.; Cyclo- 

 tella, sp. ] Orthosira punctata, Sm. ; Stephanodiscus, sj). ; Cocconema cymhiforme, 

 Ehi". 1 Navicida ajfinis, Bhr. ; Navicula, sp. ; Pinnularia -major, Sm. ; P. viridis, 

 Sm. ; Pinnularia, 2 sp. ; Pinnularia interrupta, Sm. ; Stauroneis phoenicenteron, 

 Ehr. ; and two others, the genera of which I have not been able to determine. 

 These diatoms decidedly prove the fresh water origin of the loam. 



This loam generally rests on a tolerably even surface of tlie puuiice sand, 

 but in places a good deal of contemporaneous erosion and filling up, and 

 re-arrangement of the pumice sand, ai)pears to have been going on during its 

 deposition. 



