354 C. K. LEITH AND W. J. MEAD 
comes within our range of observation. No sooner does it crystal- 
lize than changes begin—with great rapidity nearest the surface, 
with less rapidity below. ‘These changes are both chemical and 
physical. Water, carbon dioxide, oxygen, and other substances of 
the hydrosphere and atmosphere attack the rocks. The ferrous 
iron of the igneous rocks combines with oxygen and water and a 
large part of it remains as residual limonite: Alkalies and alkaline 
earths form soluble salts and are leached out in well-determined 
order. Free quartz is less readily changed or dissolved. The por- 
tions of the bases which the waters have failed to abstract tend to 
remain combined with alumina and silica, taking on water to form 
new hydrous silicates, although some of them may remain in place 
as carbonates or other salts. The excess of alumina and silica left 
from the leaching out of the bases from silicates becomes hydrated 
and forms clay. ‘There are ultimately left, then, iron oxide, quartz, 
clay, and a variety of hydrous aluminum silicates, characterized 
by a lower ratio of silica to the bases than in the original silicate 
minerals. Oxide zones of ore deposits are special cases of these 
residual products. ‘These substances on the erosion surface are 
mechanically distributed and ultimately become segregated as mud 
and sand, or even iron ores. The substances which are taken out in 
solution may remain in solution in the sea or may be redeposited 
as limestone, dolomite, chert, iron carbonate, or iron silicate—in 
other words, as chemical sediments. Part of these substances also 
may be deposited as cements. The ultimate results of the de- 
struction of the original igneous rocks, then, are the sediments, 
the oceanic salts, and the non-transported residual products of rock 
decay. 
The changes above outlined involve the addition of substances 
of light molecular weight, such as carbon dioxide, water, and oxygen. 
In terms of elements oxygen is of dominant importance. The 
resulting minerals are on the whole of simpler molecular composi- 
tion and of lower specific gravity. The actual addition of new 
substances has also increased volume. The unconsolidated prod- 
ucts have a large pore space which still further increases the 
volume of the mass. Energy is released in an enormous amount. 
It has been calculated that a gram of average igneous rock releases 
on complete decomposition at least 120 calories of heat. 
