ae 
T. S. Hunt on the Chemistry of Natural Waters. 181 
removed by substitution from sedimentary strata are those 
_ which are liberated by the slow decomposition of the minerals 
composing these sediments. 
t has long been known that in the transformation of a feld- 
spar into kaolin the double silicate of alumina and alkali takes 
up a portion of water, and is resolved into a hydrous silicate of 
umina; while the alkali, together with a definite portion of 
silica, is separated in a soluble state. The feldspar, an anhydrous 
double salt formed at an elevated temperature, has a tendency 
under certain conditions to combine at a lower temperature with 
a portion of water, and break up into two simpler silicates. 
aubrée has moreover shown that when kaolin is exposed toa 
heat of 400° C. in presence of a soluble silicate of potash the 
ftom the surface. ‘This chemical alteration, according to Four- 
net, is always preceded by a mechanical change of the feldspar, 
which first becomes opaque and friable, and is thus rendered per- 
meable to water. He conceives this alteration to be molecular 
and to be connected with the passage of the silicate into a dimor- 
phous or allotropic condition.’ 
§ 12. The researches of Ebelman in the alterations of various 
Tocks and minerals have thrown considerable light on the rela- 
Kons of sediments and natural waters.’ From the analyses of 
basaltic and similar rocks, which include silicates of lime, mag- 
dec : 
moved, together with a large proportion of silica. It was 
nd moreover that in the case of a rock a parently com 
of labradorite and pyroxene, the removal of the lime and mag- 
isla from the decomposed portion was much more complete 
than that of the alkalies; showing thus the comparatively greater 
ability of the feldspathic element. The decomposition of the 
the final result approxi ilicate of alumina, or 
ximates to a hydrous silicate | ‘ 
clay. This aa. eecanaienita of silicates of protoxyd-bases 
*ppears to be due to the action of carbonic acid, which, remov- 
* o: " 
Ann. de Chimie, [2], Iv, 295. « Ebelman, Recueil des Travaux, ii, 1-79, 
y *%. Jot Bei. —seconp Series, Vou. XXXIX, No. 116.—Mancu, 1865. 
