504 ME. P. ETJTLEY ON [Aug. I9OO, 



walls of some of these cavities renders such an assumption more 

 than probable. The optical character of the substance almost 

 wholly constituting this section agrees perfectly with that pre- 

 viously described as characteristic of geyserite. This specimen 

 indeed seems to be a remarkably pure example, since after powdering 

 and drying it at 100° C, Mr. Philip Holland found the percentage of 

 silica to be 93*59. Some idea of the probable nature of the other 

 constituents may be gathered from an analysis, by J. W. Mallet, of 

 & siliceous deposit from the hot springs of Lake Taupa : l 



Per cent. 

 Si0 2 94-2(T 



A1 2 3 1-58 



Fe 2 3 0-17 



CaO trace 



NaOl 0-85 



HJ3 306 



: 99-86 



In one of the more translucent bands in this section there may 

 be found evidence of diatoms, apparently similar to those com- 

 monly met with in deposits of tripoli. An amplification of about 

 250 diameters is, however, necessary before the existence of these 

 small bodies can be recognized. 



'& j 



H 4l . R o t or u a. — A compact semi-opal, with waxy to sub-vitreous 

 lustre and couchoidal fracture. It has a rude, irregularly -banded 

 structure, one band being reddish-brown and the rest bluish-grey. 



Under the microscope, the section is mainly isotropic between 

 crossed nicols, and exhibits no appreciable structure. A few specks, 

 apparently of pyrites, and numerous doubly-refracting grains, without 

 any definite crystal-boundaries or cleavages, are present. 



The reddish portions apparently owe their colour to haematite in 

 a very fine state of division. A very obscure fibrous structure may 

 be present in places. 



H 42 . Eotorua. — Rock consisting of white fragments, most of 

 them apparently felspathic, embedded in a matrix or cement which 

 is in part light bluish-grey and crystalline, in part dark or almost 

 black. The specimen looks like a rather coarse breccia. On a 

 cut surface, some of the white fragments show very small cavities, 

 generally elongated and suggestive of irregular furrows. 



Under the microscope, the section is seen to consist chiefly of 

 lapilli of pumice, a few fragments of rock, possibly rhyolite, some of 

 these being opaque and pink, others reddish-brown in transmitted 

 light, and all more or less corroded superficially. There are also 

 fragments of crystals, upon the original nature of which it is 

 impossible to speculate, since they do . not show definite crystal- 

 boundaries or any cleavages : they are traversed by irregular- 

 cracks, and are practically isotropic. The section also contains a 

 fragment of quartz. The whole of these fragments are bordered by 

 hyalite, the remainder of the rock being filled in by isotropic silica 



1 Phil. Mag. vol. v (1853) p. 285; also quoted in Hochstetter's 'New 

 Zealand' transl. E. Sauter (Stuttgart, 1867) p. 435. 



