822 Transactions. — Geohfij/. 



width of about 15 miles between the valleys of the Mokau and the 

 Ongarue Kivers. The high ground of the plateau is covered with 

 forest, and the soil is of good quality ; the valleys are open — pro- 

 bably the result of continued denudation ; their bottoms are for 

 the most part covered with deposits of pumice sands, and conse- 

 quently poor and unfit for cultivation as a rule. The long sinuous 

 spurs of the plateau are composed of rhyolites, andesites, and tuff, 

 the latter predominating, and with it are interbedded masses of 

 volcanic agglomerate, and enfolding with them rounded pebbles of 

 igneous rocks, and those of slate rocks also. At their base are 

 beds of marl, shale, and sandstone ; in what position these beds 

 lie relatively I cannot say with any certainty — I never had the 

 time nor opportunity to trace out their junctions properly. I 

 think the beds of marl and shale rest conformably on the sand- 

 stone, as they appear to do in one of the deep-worn banks of the 

 Mapui Stream near the Mokau. The tuff beds overlie the marls, 

 but whether conformably or not I cannot say ; the colossal 

 masses of tuff rise up one after another, bordering the valleys ; 

 isolated peaks and blocks of it stand out on the ridges, with here 

 and there greatly denuded sides. 



It would seem that the plateau was formed of a continuous 

 sheet of the formation of which these blocks and ridges are the 

 remains, and the softer portions being carried away by denu- 

 dation, the river valleys were formed, and varieties of contour 

 worked out all over the plateau. The beds of tuff contain large 

 quantities of pumice in various sizes, up to a foot in diameter. 

 Some of these are abraded and rounded at the edges, others are 

 sharp and angular. I did not notice any fossils in the beds. 

 These tuff beds much resemble those of the Patetere plateau, to 

 be seen between Waotu and Lichfield ; they have a thickness 

 of probably 1,200 feet on the plateau just described. 



A reference to the section will show that the plateau rises with 

 a gradual and regular slope of 3 degrees, from the Mokau Valley 

 eastward to a height of 2,300 feet on the Tangitu Eange ; and 

 then there is a sudden fall of 1,600 feet into the valley of the 

 Ongarue. If the line of slope be still produced in an easterly 

 direction, it will meet the summit of the main range at Hauhu- 

 ngaroa, 12 miles to the west of LakeTaupo, and from there the land 

 slopes into the Taupo basin at an angle of 14 degrees. 1 have 

 dwelt considerably on the description of this tufa deposit, for the 

 reason that it occupies the largest area of any formation in the 

 district, and is persistently associated with all the other for- 

 mations, but in what relation to them my investigations were 

 not complete enough to show. I believe that in the Mokau 

 Valley, and in the plateau between it and the Ongarue, the tuff 

 beds lie conformably on the tertiary limestones and calcareous 

 clay marls ; but their relation to the marl beds in the Ongarue 

 and Upper Whanganui valleys is, so far as my knowledge goes, 



