318 Transactions. — Geology. 



from north to south, and from 6 to 8 miles wide. Sur- 

 rounded hy lofty mountains from 2,000 to 3,700 feet above 

 the sea, the valley drops almost suddenly to 600 feet above sea- 

 level. The drainage area of this basin is comparatively small, 

 only about 250 square miles. It is bordered in many places by 

 steep coast-like cliffs of tuffs, andesite, and agglomerates, stand- 

 ing out as greatly denuded fragments of the masses which lie 

 at the back of them. The valley itself is partly filled up by the 

 later-formed deposits of pumice and detritus carried down from 

 the hillsides by the surface waters and running streams. 



It would seem almost incredible that this great deep valley 

 owes its origin to the ordinary denuding agencies, and it is more 

 probable to be partly the result of a great earth fissure, con- 

 nected with the volcanic movements to the eastward, and pos- 

 sibly a syncline in connection with the Hauhungaroa Kange. It 

 may also be remarked that the Hauhungaroa Range runs 

 parallel with the great fissure of the Taupo volcanic zone, thus 

 apparently showing a sequence of the volcanic operations by 

 which the mountain chains and valleys of other parts of the 

 world are formed. 



The Tuhua Mountain is a very prominent and interesting 

 feature in this part of the country. It stands as an isolated 

 mountain on the eastern side of the Ongarue Valley, and 6 

 miles to the west of the main range. Its height above the 

 sea is 3,425 feet. It is thus quite as high as the main 

 range, which it stands 6 miles away from. On the summit 

 rest large rounded boulders of tufaceous sandstone over 20 

 feet in diameter. The sides are flanked by a silt-like marl 

 deposit (to be referred to again). On the top is a broad flat 

 platform, over 00 acres in extent. On the south and north 

 the mountain sides are very steep, and in some places quite 

 precipitous, so that vast quantities of material have rolled 

 down into the valleys in extensive landslips, evidences of which 

 both old and recent are to be seen on the southern, western, 

 and northern slopes of the mountain. Hikurangi Mountain is 

 also an interesting feature of the valley, and from a geological 

 point it is of much importance. It is formed of soft tufaceous 

 rock, with hard rhyolitic rock interbedded with it. The sides of 

 the mountain are flanked with brown and blue clay marls, and 

 probably the base is conrposed of masses of the same rocks, as 

 from near the base, at an altitude of 800 feet above the sea, two 

 dense and powerful mineral springs appear. These are highly 

 charged with salt, and are of considerable importance from their 

 medicinal properties, as disclosed in the analysis by Professor 

 F. D. Brown, and referred to in his paper, " Notes on a Salt 

 Spring in the King Country," read before the Institute on the 

 14th of November, 188G. Hikurangi looks at a little distance 

 away from it like a very regular volcanic cone, rising from a 



