Thomas. — Geology of Tongariro and Taupo. 343 



of the area, whilst elsewhere around the margin the ground 

 dips down abruptly to the steep outer slope of the cone (see 

 Plate XXX). The two lengths of cliff and a few other rocks in 

 the same ch'cle evidently mark the former rim of a crater, 

 which at a subsequent period was filled up by lava, which 

 overflowed the brim and formed streams on the rocky sides of 

 the cone. From its position we may distinguish this fiUed-up 

 crater as the North Crater of Tongariro. On the west side, 

 and just within its margiir, it contains a smaller funnel-like 

 crater of considerable depth. The cliff to the south is chiefly 

 formed of thick horizontal beds of dark lava. At the western 

 angle of the cliff the smooth and slightly-weathered surfaces 

 of the joints in the lava show a remarkable streaky structure, 

 visible from a great distance. This flow-structure, which beais 

 testimony to the unequal movements in the lava at the time 

 of its consolidation, is due to the irregular alternation of a 

 light- and dark-grey material in the ground-mass of the lava. 

 Near this spot the surface of the ground is covered with a 

 layer of an exceedingly light pumice of acid composition, 

 differing greatly from the other rocks found on the mountain. 

 The largest of the fragments was 14in. in its longest diameter. 

 Eeference to this and other rocks on Tongariro will be found 

 further on. 



Between this North Crater and the rest of the Tongariro 

 system is a dip of 200ft. or 300ft., by which it is marked off 

 from the rest of the mountain-top. To the south, and stretch- 

 ing as far as the slope of Ngauruhoe, is another cone, marked 

 by a very large crater, over half a mile in length. This crater, 

 w^hich we may suitably distinguish as the South Crater, is 

 of a much elongated form, and it is worthy of note that its 

 long axis coincides in direction with the Tongariro-Ruapehu 

 line. 



Its walls are very steep, and in many places precipitous. 

 They are highest at the end towards the North Crater, and 

 gradually diminish in height in the direction of Ngauruhoe. 

 The highest part of the crater- wall, which is also the highest 

 point on Tongariro, lies on the western side, and is about 

 6,450ft. above the sea-level. The crater-flow lies over 800ft. 

 below this point. At times the bottom of the crater must be 

 covered in part by a shallow lake, which discharges at the 

 end towards Ngauruhoe. The water will pass down the 

 watercourse between the latter mountain and Tongariro, and 

 form the beginning of the Mangatepopo Stream, a tributary of 

 the Wanganui Eiver. 



Travelling along the ridge which forms the eastern boun- 

 dary of the South Crater, we pass two large craters on 

 the right. The first of these has high precipitous sides 

 towards the west, whilst to the east its wall is wanting, 



