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T. S. Hunt—The Decay of Rocks. — 195 
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Essays, I have pointed out the important part played by th 
protoxyd-bases liberated by the, sub-aérial decay of feldspathic 
and hornblendic rocks. Starting from the conception of a. 
primitive terrestrial crust consisting wholly of crystalline sili- 
cated rocks, we are forced to find in such a process of decay 
the source of all limestones and dolomites, which are derived 
from the carbonates of li 
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the former which is seen when silicates like orthoclase and 
albite are compared with micas like muscovite, and with sili- 
cates like cyanite, pyrophyllite and staurolite. The conclusion — 
was then reached that ‘the chemical and mineralogical consti- 
tution of different systems of rocks must vary with their — 
antiquity,” and that “it now remains to find in their compara: 
tive study a guide to their respective ages;” in which connec- 
tion a comparison was then attempted between the older 
gneisses and the newer crystalline schists. A further applica-~ — 
tion of this principle was essayed in 1878, when the progres- 
