Hill. — On Distribution of Pumice. 293 



passes through them most readily, and must move in under- 

 ground courses until forced to the surface by the filling and 

 overflowing of some underground basin. Everything is favour- 

 able to the existence of similar springs at the junction of the 

 pervious and impervious rocks in this district ; and, if we add 

 the quantity of water likely to percolate through the artesian 

 shingle-bed along its line of outcrop, situated some distance to 

 the west of Maraekakaho, we may easily account for the existence 

 of that wonderful supply of pure well-filtered water which now 

 passes underneath the plain, and which has proved of so much 

 real worth from a sanitary point of view, as well to the people of 

 Napier as to those residing in the surrounding district. 



I append sections (Plates XVI. and XVII.) showing the 

 inclination of the plain in various directions, also showing the 

 depth of the ocean for a distance of 16 miles out in the bay. 

 The deepening of the artesian bed is also shown between Paki- 

 paki and Napier. 



The sections fully illustrate the points referred to in this 

 paper. 



Art. XXXIX. — Pumice : Its Geological Distribution on the East 

 Coast of the North Island of New Zealand, extending from 

 TologaBay (38° 20' S. lat.) to Cape Turnagain (40° 30' S.). 



By H. Hill, B.A., F.G.S. 



[Read before the Hawke's Bay Philosophical Institute, 14th November, 1887.] 



Plate XVIII. 



In a short paper which I had the honour of reading before this 

 Society last year, on " Traces of Volcanic Dust- Showers at 

 Napier, and generally throughout the East Coast District,"* I 

 mentioned, incidentally, that I had been engaged for some time 

 past in collecting data as to the age and the extent of the pumice 

 deposits so common throughout the district. I now beg to submit 

 the results of my investigations as far as I have yet gone. 



Pumice is a substance essentially volcanic in its origin, and 

 is always more or less common in volcanic districts. It belongs 

 to the class of volcanic rocks known as vitreous, as distinguished 

 from crystalline, and to the sub-class acidic, as distinguished 

 from basic. Kocks are said to be acidic when they contain as 

 part of their constituent ingredients more than 60 per cent, of 

 silica. "When they contain less than 60 per cent, of silica 



* " Trans. N.Z. Inst.," vol. xix., p. 385. 



