294 Transactions. — Geology. 



they are said to be basic. According to Jukes, the following 

 substances enter into the composition of pumice. The maxi- 

 mum, minimum, and mean percentage of each substance is 

 given :— 



Maximum Minimum Mean 



Substance. percentage. percentage. percentage. 



Silica 77 ... 61 ... 68-1 



Alumina 18 ... 10 ... 14- 



Potash 6 ... 1*5 ... 3-7 



Soda 11 6 



Magnesia 1 ... ... ... *6 



Oxide of Iron and 



Manganese ... 4-5 ... 0-5 ... 3.2 



From this list of constituents it will be seen that, notwith- 

 standing the generally supposed sterility of pumice, it contains in 

 its composition every essential product needful for the making 

 of what is known as a fertile soil. Indeed, I have often thought 

 that a good deal might be said in support of the claims of 

 pumice as being the origin, in this district at least, of most of the 

 fertile soils. By far the greater portion of a mass of pumice is 

 composed of silica and alumina, which, in a state of chemical 

 combination, form clay. The same mass contains soda and 

 magnesia, which form a part of every rich soil ; and the same 

 mass also contains potash, which is essential to the fertility of 

 all soil. Liebig, in his " Chemistry of Agriculture and Physio- 

 logy," says that a Hessian acre of disintegrated felspar 20 inches 

 deep contains 1,152,000 lbs. of potash. This would amount to 

 1,120,000 lbs. in an English acre 12 inches deep. Now in 

 pumice there is about one-fourth the quantity of potash as there 

 is in the same weight of felspar, so that in an acre of disin- 

 tegrated or decomposed pumice 12 inches deep there must be 

 something like a quarter of a million pounds of potash, whilst the 

 quantity of soda in the same disintegrated mass would weigh 

 nearly 400,000 lbs. The percentage composition of an ortho- 

 clase felspar, according to Rutley, is S x 2 = 04-20, Al 2 3 = 

 18-40, K 2 = 1G-95. I am not aware of any special analytical 

 tests having been made in New Zealand as to the composition 

 of pumice, but I am inclined to the opinion that the pumice 

 found throughout the East Coast of this Island contains less 

 silica and more alumina than the amounts stated in the table 

 quoted above. 



Pumice, such as is to be met with in large quantities along 

 the sea-beach of Hawke's Bay, is a light spongy-looking kind of 

 rock which swims upon the surface of water. In reality, pumice 

 is a porous or vesicular glass, closely allied in its composition to 

 obsidian, and, in fact, it is found on the surface of obsidian lava- 

 flows in volcanic districts generally. It must, not, however, be 

 supposed that pumice is always to be found on the surface of 



