75 
1920.] Engineering Survey of Proposed Arapuni Dam Site. 
so-called ‘ wilsonite ’ of the Waihi district. It has a fine-grained matrix, 
in which small fragments and flakes of pmniceous 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.’’ This resemblance to the 
rock of the Waihi district has also been commented on by Mr. Gauvain, 
chief engineer to the Waihi Gold-mining Company, in conversation with 
the writer on occasions of visits to Arapuni. The andesite, obsidian, grey- 
wacke, and white mudstone, also described by Dr. Henderson in his above- 
quoted article, are encountered in and behind the boulder formation near 
the river-edge at the 190 ft. level (drives Nos. 3 and 4). 
ARAPUNI 
Fig. 2.—Plan and section of prospect drives and shafts at dam site. 
The western shafts are completed throughout from the top of the cliff 
(328 ft. level) to 20 ft. below the river-bottom (126 ft. level)—just over 
200 ft. vertical in all. The well-consolidated nature of the rock penetrated 
is of particular interest in view of two theories advanced as to the origin¬ 
ating cause of the Arapuni Gorge. One, not favoured by the geologists, is 
that it is the result of an earthquake crack, and, that being so, clefts and 
fissures similar to those showing on the laminated face would occur farther 
back from the cliff-face and under the river. The other theory is that the 
gorge is the result of normal river erosion, and that faults in the rock are 
not likely to be encountered. That it appears now as if the latter view 
