

374 STRUCTURAL AND FIELD GEOLOGY 
felspar and mica not only worn, but to some extent chemically 
altered—nevertheless, each mineral has retained its individu- 
ality... In like manner, certain of the minor accessory 
ingredients of crystalline igneous rocks have often survived 
the demolition of their parent rocks, and are now and again 
met with as rolled pebbles and grains in sand and gravel. In 
such cases, however, the gravel and sand have usually been 
derived directly from the disintegration of the igneous rocks 
in question. Neither zircon, rutile, nor magnetite could 
survive the manifold vicissitudes through which grains and 
pebbles of quartz have passed. Sooner or later they lose 
their individuality, and are transformed. 
Thus the process of rock-disintegration or “weathering,” 
as it is termed, may be said to consist essentially in the 
breaking up of complex, and therefore usually unstable 
compounds, and the consequent production of simpler and 
more stable bodies. Hence, when igneous and _ schistose 
rocks are highly weathered, their complex silicates are trans- 
formed and converted into simpler compounds, some of which 
are soluble, while others are more or less insoluble. The 
soluble ingredients tend, therefore, to be leached out and 
washed away. The soil-cap formed upon such rocks, how- 
ever, is rarely quite destitute of soluble matter. Even when 
the disintegrated materials have been transported by water 
and deposited elsewhere in the form of sand, clay, silt, etc., 
these sediments will usually retain a larger or smaller 
proportion of soluble matter—the process of disintegration 
and decomposition of the several constituents of the original 
or mother rock has not been completed. In short, sedi- 
mentary deposits, derived directly from the breaking-up of 
igneous masses, frequently contain a larger or smaller 
proportion of the relatively unaltered detritus of the parent 
rocks. We know, however, that many sedimentary rocks are 
built up of materials which have been used over and over 
again. Rocks of this kind, therefore, may consist exclusively 
of insoluble ingredients—-the only soluble matter they may 
contain being the binding or cementing material introduced 
inte them by percolating water. Repeatedly exposed to 
weathering—winnowed and rewinnowed again and again by 
wind or water, or both—sedimentary materials eventually 

