BY PROFESSOR STEPHENS, M.A., F.G.S., &C. 517 



has maintained a certain amount of activity up to the present 

 time. From this centre the Waikato, the Wanganui, and the 

 Wangahu rivers take their respective courses. The north-western 

 sides of this valley are formed by the edge of a vast plateau of 

 trachytic lava, emitted probably from fissures in the first instance, 

 though subsequently penetrated by a few cones of eruption. 

 These appear to be more frequent in the less elevated portions of 

 the plateau, and on the isolated patches which rise near its 

 margin, a fact which may indicate a very great thickness in the 

 central portion of this trachytic area. The whole plateau is 

 densely wooded, and intersected with deep valleys, along two of 

 which the Waikato and Wanganui make their way in divergent 

 directions. To the N.W., but quite separated from these trachytes 

 is the Pliocene basaltic plateau of the Lower Waikato. The 

 former region therefore appears to be occupied by the oldest 

 volcanic rocks which are to be found in the wifle space between 

 the Maitai rocks to the S.E., and their re-appearance on the 

 Thames. And it would seem that these trachytes underlie the 

 pumice beds of the Taupo Zone throughout, and that these latter 

 just conceal the line of contact of the former with the aforesaid 

 Maitai carboniferous rocks. For at both ends of the Taupo Lake, 

 and in like manner to the N.E. of Tarawera, this trachytic 

 formation emerges in insular patches from the pumice, supporting 

 in each case true cones of eruption which have been subsequently 

 thrown up through it. These are all represented by Hochstetter 

 as having completed their periods of activity by forming a central 

 cone or plug of a more silicious lava, rhyolite, within the crater 

 of each. The same rock forms a margin to Lake Taupo, and 

 further occupies the whole area of the Lake district. 



Now this rock is of the same materials as Obsidian, which is 

 its glassy condition, cooled rapidly, so as to prevent the separation 

 of the constituent minerals, and under pressure, so as to 

 compress whatever gaseous matters it might contain from normal 

 expansion. Pumice on the other hand is the same material 

 charged, in its original mass, with an enormous proportion of 

 compressed gas (steam) which expanding as the lumps of liquid 



