74 
The N.Z. Journal of Science and Technology. [April 
for a distance of about 60 chains. Arrived here, opposite the proposed 
power-house site, situated in the bottom of the present gorge, an overflow 
weir across the old channel will turn the river into parallel steel-lined tunnels 
about 7 chains long, piercing the above-mentioned fern-covered ridge at 
a very steep inclination, and delivering the water to the machines with a 
total head, measured from the crest of the spillway weir, of about 165 ft.— 
25 ft. of which represents the head gained by the fall in the gorge between 
the dam site and the power-house site. Hence the selection of the power¬ 
house-site so far below the dam—an arrangement which is rendered most 
economical by nature’s provision of the natural head-race already referred 
to. When the works are installed the present gorge will thus be dry 
between the dam and the power-house, the diverted water taking up its 
present course again on leaving the machines immediately below the power¬ 
house. Fig. 3 gives a view of the gorge at the dam site, and the mouths of 
some of the prospecting-drives can be seen. 
There will be no overflow at the main dam, which will be built to a height 
10 ft. above the crest of the spillway weir, where all overflow will be dealt 
with, this overflow re-entering the present channel about a mile farther 
on by way of the old watercourse. 
To proceed with the description of the examination of the dam site 
now in hand : It is required rigidly to examine and test the material in and 
on which the dam is to be built, for compression, watertightness, porosity, and 
power to resist erosion by water under pressure. It is obvious, therefore, that 
sufficient of the site foundations and surrounding country should be opened 
up, probed into, and tested to obtain thoroughly representative tests over 
the whole site. To do this a series of shafts is being sunk on each' side of 
the gorge from the top of the cliffs to 20 ft. below the bottom of the river, 
and connected by a tunnel under the river. Nine prospecting-drives (five 
on the eastern bank and four on the western) have been driven in from 
the face of the cliff to these shafts, or outwards from the shaft where the 
perpendicular cliff-face makes access from outside difficult, at approximately 
every 30 ft. between water-level and the top of the cliffs., In addition, 
eight short cross-drives have been driven across the site to test the country 
to each side of the main drives. 
On referring to fig. 2 it will be seen that the eastern shafts are at present 
completed for 127 ft. from the top of the cliff (347 ft. level) to No. 2 drive 
(220 ft. level), penetrating a light-coloured well-consolidated rock, prac¬ 
tically free from fissures, clefts, or shakes. The lower portion of these shafts 
was driven upwards to meet that descending from the surface. At level 
238 ft. the perpendicular cliff ceases and a deep talus overlying easier slopes 
occurs. 
Drive No. 2 (220 ft. level) was driven in for 37 ft. through this talus, 
and then on under the solid cliff for a farther 160 ft., penetrating at first a 
dark volcanic tuff which grades upward into the light-coloured well-consoli¬ 
dated rock above referred to, and inward to a grey or buff-coloured rock 
in which pumiceous rhyolite is well marked. This latter formation under¬ 
lies the talus on both sides of the river, and grades gradually upward under 
the main cliff into the light-coloured rock first mentioned. 
Dr. Henderson, Government Geologist, in his “ Notes on the Geology 
of the Waikato Valley near Maungatautari ” (this Journal, vol. 1, No. 1, 
p. 58), refers to the upper rock as tc a light-coloured well-consolidated rock, 
and on page 56 describes what is without doubt the lower rock as “ a light- 
coloured rock of subaerial origin which bears a marked resemblance to the 
