488 ADDRESS OF T. STERRY HUNT. 
soda, leaving behind silica, alumina and potash — the elements of 
granitic, gneissic and trachytic rocks. The finer and more alumi- 
nous sediments, including the ruins of the soft and easily abraded 
silicates of the pyroxene group, resisting the penetration of the 
water, will, on the contrary, retain their alkalies, lime, magnesia 
and iron, and thus will have the composition of the more basic 
rocks. * 
A little consideration will, however, show that this process, al- 
though doubtless a true cause of differences in the composition of 
sedimentary rocks, is not the only one, and is inadequate to ex- 
plain the production of many of the varieties of stratified silicated 
rocks. Such are serpentine, steatite, hornblende, diallage, chlorite, 
pinite and labradorite, all of which mineral species form rock-masses 
by themselves, frequently almost without admixture. No geologi- 
cal student will now question that all of these rocks occur as 
members of stratified formations. Moreover, the manner in which 
serpentines are found interstratified with steatite, chlorite, argillite, 
diorite, hornblende and feldspar rocks, and these, in their turn, 
with quartzites and orthoclase rocks, is such as to forbid the notion 
that these various materials have been deposited, with their present 
composition, as mechanical sediments from the ruins of preéxist- 
ing rocks; a hypothesis as untenable as that ancient one which 
supposed them to be the direct results of plutonic actiop. 
There are, however, two other hypotheses which have been pro- 
posed to explain the origin of these various silicated rocks, and 
especially of the less abundant, and, as it were, exceptional species 
just mentioned. The first of these supposes that the minerals of 
which they are composed, have resulted from an alteration of pre- 
viously existing minerals, often very unlike in composition to the 
present, by the taking away of certain elements and the addition 
of certain others. This is the theory of metamorphism by pseu- 
domorphic changes, as they are called, and is the one taught by 
the now reigning school of chemical geologists, of which the 
learned and laborious Bischof, whose recent death science deplores, 
may be regarded as the great exponent. The second hypothesis 
supposes that the elements of these various rocks were originally 
deposited as, for the most part, chemically formed sediments, OF 
precipitates ; and that the subsequent changes have been simply 
od yi 
Quar. Jour. Geol. Soc., xv, 489; also, Amer. Jour. Sci., II, xxx, 133. 
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