58 The N.Z. Journal of Science and Technology. [Jan. 
Notes on the Geology of the Waikato Valley near 
Maungatautari. 
By J. Henderson, D.Sc., Mining Geologist, Geological Survey Department. 
(A map illustrating this paper appears in the Eleventh Annual Report of the 
New Zealand Geological Survey, 1917.) 
The Waikato River, to the south-east of the irregular volcanic cone of 
Maungatautari, flows in a north-north-easterly direction, this portion of 
its course forming the Arapuni Gorge. Escaping from this gorge, its 
course is due north in a well-terraced but narrow valley for seven miles, 
and then the river, turning abruptly to the left, enters the Maungatautari 
Gorge, through which it flows nearly west for five miles. Near this point, 
about five miles from the town of Cambridge, the river reaches the middle 
Waikato basin, which may be considered part of the plain that stretches 
northward to the Hauraki Gulf. The river from its entrance of the Arapuni 
Gorge to its exit from the Maungatautari Gorge, a distance of about eighteen 
miles, falls nearly 200 ft. 
The rocks exposed along the valley as far as examined are almost entirely 
of a pumiceous nature, and range from well-consolidated tuff to loose sand. 
For about a mile at the upper end of the Maungatautari Gorge greywackes 
and argillites, considered to be of Trias-Jura age, outcrop along the bed 
and north bank of the river. These do not appear to rise more than 50 ft. 
above the level of the stream. Similar rocks are reported to occur on the 
Puketurua-Putaruru Road two or three miles west of the latter township. 
At the Maungatautari Gorge the irregular surface of the old rocks is overlain 
by pumiceous tuff, which in this locality forms the crests of hills that rise 
750 ft. above the stream-level. Similar material outcrops at many points 
along the river, and on either side of the valley forms hills rising to heights 
of a like order. 
Much of the tuff, which varies considerably in colour and texture, was 
evidently laid down under water. Prominent, however, is a light-coloured 
rock of subaerial origin which bears a marked resemblance to the so-called 
x ‘ wilsonite ” of the Waihi district.* It has a fine-grained matrix, in which 
small fragments and flakes of pumiceous rhyolite lie roughly parallel. This 
rock in fresh section is usually a pale-yellow, grey, or buff colour, and 
weathers to various shades of grey. In some localities rounded pebbles of 
andesite, obsidian, and greywacke occur ; in others the pumice fragments 
increase in size and number till the rock becomes a pumice breccia, while 
in others the larger flakes may entirely disappear. Sometimes the matrix 
is darkened by the presence of mica flakes, an effect probably due to the 
sorting action of water. These, however, may be altogether wanting when 
the rock becomes a gritty sandstone. Although in general the material of 
the tuffs is obviously of pyroclastic origin, a few bands occur in which the 
form of the fragments has been decidedly modified by water-action. Thus 
beds of coarse and fine pumiceous conglomerate, well-bedded sandstone, 
and white mudstone are found intercalated between thick layers of tuff and 
breccia.. 
The fine material that constitutes the bulk of the rock consists almost 
entirely of minute fragments of volcanic glass. These readily decompose 
* See J. Henderson, N.Z. G.S. Bull. No. 16, 1913, pp. 70-73, and previous 
literature there cited. 
