ORIGIN AND FORMATION OF SOILS. 133 
with hydrated silica, and ofien with grains of undecompos- 
ed mineral. If we compare the composition of pure pot- 
ash feldspar with that of kaolinite, assuming, what is 
probably true, that all the alumina of the former remains 
in the latter, we find what portions of the feldspar have 
been removed and washed away by the water, which, to- 
gether with carbonic acid, is the agent of this change. 
Feldspar. Kaolinite. Liberated. Added. 
ANUMAIAD ces iors 6 18 3 18 3 0 
STL a ee 64.8 23.0 41.8 
IPOUAS Biro. vo ooo, 3 16.9 16.9 
ERE ge eae es 6.4 6.4 
100 47.7 58.7 64 
It thus appears that, in the complete conversion of 100 
parts of potash feldspar into kaolinite, there result 47.7 
parts of the latter, while 58.7°|, of the feldspar, viz: 
41.8°|, of silica and 16.9°|, of potash, are dissolved out. 
The potash, and, in case of other feldsp:rs, soda, lime, 
and magnesia, are dissolved as carbonates. If much water 
has access during the decomposition, all the liberated silica 
is carried away.* It usually happens, however, that a por- 
tion of the silica is retained in the kaolin (perhaps in a 
manner similar to that in which bone charcoal retains the 
coloring matters of crude sugar). The same is true of a 
portion of the alkali, lime, and oxide of iron, which may 
have existed in the original feldspar. 
The formation of kaolin may be often observed in na- 
ture. In mines, excavated in feldspathic rocks, the fis- 
sures and cavities through which surface water finds its 
way downwards are often coated or filled with this sub- 
stance. 
ec. Other Silicious Minerals, as Leucite, (Topaz, Scapo- 
lite,) ete., yield kaolin by decomposition. It is probable 
that the micas, which decompose with difficulty, (phlogo- 
* We have seen (H C. G., p. 121) that silica, when newly set free from combi- 
nation, is, at first, freely soluble in water. 
