ee me BOTANY 
minutes and then pour off the liquid. Repeat this 
process till the water comes off clear. Dry and weigh 
the residue. The loss of weight in grams gives the 
percentage of the smaller particles, chiefly clay and 
silt. Now fix the sieves on a stand one above the other, 
the largest mesh on top, and the rest in descending 
order of size. Place a tray below the bottom sieve. 
Place the dried residue on the topmost sieve and 
rub it with the fingers. Stones will be left on the 
5 m.m. sieve, coarse gravel on the 3 m.m., fine gravel 
on the 1 m.m., coarse sand on the -2 m.m., and fine 
sand on the tray. The different grades may now be 
weighed and the percentage thus arrived at. 
By placing the various grains in a long glass 
tube, the smallest at the bottom and the rest in 
ascending order, a permanent and instructive record 
is secured. (Fig. 53). 
The nature of the particles in any soil depends on 
the nature of the rock from which it is derived and 
the extent to which that rock is decomposed or 
broken up. 
Rocks which, like granite, contain, among other 
things, particles of quartz or silica, are by various 
agencies broken down into a mixture of sand and clay. 
The particles of silica go to form grains of sand, for 
silica, unlike most rock-forming minerals, does not 
readily decompose. Such a soil, when supplied with 
humus, constitutes aloam. Rocks, which, like the 
basalts of the Auckland Peninsula, contain no free 
silica, eventually decompose into pure clays. The 
clays of the northern gum lands, though transported. 
by water from the localities where they were formed, 
evidently originated in this way. 
The so-called voleanic soils of the same districts 
consist really of decomposed, mixed with pleces of 
undecomposed basalt. 
