502 MR. F. RUTLEY ON [^-Ug. I9OO, 



the belief that algous sinter forms a smaller proportion of the siliceous deposits 

 than it does at most of the geyser-basins of the Yellowstone, where the waters 

 are chiefly alkaline.' 



Mr. Weed gives a brief account of the microscopic characters of 

 some of the siliceous sinters of the Yellowstone, but remarks : 



' Thin sections of siliceous sinters fail to show the origin and nature of the 

 deposit as clearly as had been hoped ' ; 



and again (op. cit. p. 667), 



' If many of the algous sinters fail to reveal an organic structure beneath the 

 microscope, they are nevertheless easily distinguished from the more glassy 

 and pearly sinters formed by evaporation.' 



Again (op. cit. pp. 671-72), 



' The physical differences in the unaltered sinters formed by evaporation 

 and those of algous origin is generally quite marked, the former being trans- 

 lucent or vitreous, hard, and heavy, while the algous sinter is opaque, white, 

 and often chalk-like in appearance.' 



The description given by ¥. von Hochstetter regarding the 

 deposit formed by one of the hot springs on the Waikato is also 

 of interest, as it is applicable to some of the specimens about 

 to be described. He states that the deposit, like that of all the 

 other springs near the Waikato, is siliceous, that, when recent, 



' It is soft as gelatine, gradually hardening into a triturable mass, sandy to 

 the touch, and finally forming, by the layers deposited one above the other, 

 a solid mass of rock of a very variable description at different places, both as 

 to colour and structure. Here it is a radiated fibrous or stalky mass of light- 

 brown colour ; there a chalcedony hard as steel, or a grey flint ; at other 

 places the deposit is white, with glossy conchoidal fracture like milk-opal, or 

 with earthy fracture like magnesite.' l 



With these prefatory quotations, I may now proceed to describe 

 some of the rocks from Rotorua, chiefly collected by Mr. James Park. 



H 38 . Rotorua. — A bluish-grey rock, with feebly vitreous lustre 

 and subconchoidal fracture. The specimen somewhat resembles a 

 pale -grey pitch stone, and shows, where weathered, a whitish 

 pulverulent crust. 



Under the microscope, the rock is seen to consist of small frag- 

 ments of pumice, with a considerable sprinkling of minute fragments 

 of felspar and an occasional grain of quartz. Some of the felspar- 

 fragments show lamellar twinning and may be referred to andesine,. 

 but sanidine and oligoclase are also present. Yiewed between 

 crossed nicols, the section is seen to contain some perfectly isotropic 

 matter which consists partly of pumice and partly of a cement 

 of amorphous silica, while some feebly or partly anisotropic matter 

 also helps to form the cement. In parts it is brown in ordinary 

 transmitted light, while in reflected light it appears pink. 



The rock is essentially a pumice-tuff, cemented and indurated 

 by silica derived from the hot springs so prevalent in the district. 



1 ' New Zealand ' f ransl. E. Sauter (Stuttgart, 1867) p. 398. 



