[TYRRELL] THE GOLD OF THE KLONDIKE 39 
the rock porous. Water also exerts a considerable influence on the 
feldspars and alkaline constituents of the rocks, dissolving out their 
more soluble parts and leaving the alumina and oxides of iron behind to 
form a fine clay. 
Oxygen in the atmosphere is also carried down by water and 
is assisted to combine with many of the constituents of the rocks, raising 
the protoxides into peroxides, etc. 
Water also combines with some of the constituents of rocks to form 
hydrated compounds, and in many cases the earth or soil so formed by 
combination with water occupies a much larger space than that occupied 
by the original rock itself. 
In addition to the above more or less chemical action of water in 
the formation of loose earth, the freezing of the water contained in the 
rock itself, or included in cracks or fissures which have been formed in it, 
exercises a very powerful influence in its disruption. Some granites 
contain about half of one per cent. of their weight of water in their 
composition and quartz occasionally contains a much larger proportion 
than this. When freezing, water expands about nine per cent. and the 
force of this expansion is almost irresistible, especially against any 
pressure which may be exerted near the surface of the earth. The 
pressure exerted by such freezing is about 150 tons, or 300,000 lbs. to 
the square foot, and while the spaces occupied by the ice may be very 
small, if they are confined or tightly enclosed the expansion engen- 
dered is irresistible. 
In addition to the expansion of water with freezing, the ex- 
pansion of ice itself after cooling and on subsequent heating is very 
great, being about .00003 per cent. for every increase of one degree 
Fahrenheit. 
Organic agencies also help to loosen and destroy the surface of the 
rock. Lichens growing on it keep it damp, and not only dissolve the 
rock themselves but retain the water which settles on it so that it can 
have time to dissolve out part of its constituents. Larger plants 
send their roots into very minute fissures and with increase in growth 
wedge portions of the rock apart, and in this way break it to pieces. 
When hard rock, of which the earth is chiefly composed, has been 
broken up, loosened and softened by weathering, the material remaining 
on the surface is known as residual sand, clay ete. In some cases these 
residual deposits may themselves form workable Placers, especially 
where all the soluble carbonates and other similar rocks have been 
dissolved away by percolating waters. 
Such residual deposits are known in Southern countries as 
Laterites. 
