Royal Institution. 467 
causes that produce the decomposition and decay of the 
surfaces of rocks, and the effects resulting in consequence 
in the economy of nature. y 
The changes of temperature, of the surface, of the ac- 
tion of electricity, of air, and of water, constantly tending 
to the disintegration of the superficial parts of rocks and 
mountains,’‘and their operation, are different in different cases. 
The expansion of water in the pores or fissures of rocks 
by heat or congelation, is a physical cause of the separation 
of their parts. The solvent power of moisture exerted 
upon alkaline or calcareous matter in rocks, is another 
cause of their decomposition. The action of carbonic acid 
tpon the lime or magnesia, and of oxygen upon the iron 
they contain, produces an analogous result. Electricity, 
which is shown by experiments with the Voltaic apparatus 
to be a most powerful agent of decomposition, seems to 
assist in all these changes ; and electrical powers are almost 
constantly exhibited im the atmosphere, sometimes occa- 
sioning the magnificent and awful effects of thunder-storms; 
at other times, by slow operation, tending to produce new 
arrangements of the elements of matter. 
The primary rdcks,’‘as was’ noticed in a former lecture, 
are less ‘decomposable than the secondary; and amongst 
the primary rocks, the serpentines, micaceous schists, 
marbles, and porpliyries, aré more decomposable than the 
granites: amorigst'the secondary rocks, shale, basalt, and 
soft sand-stone, are usiially very subject to decay. 
The graiites ant’ porpliyries, by their decomposition, 
afford sand and porcélain clay ; the’serpentines and schists, 
usually a‘brown’ or ‘yellow ‘earth ; the sandstones, sand ; 
limestones, ‘marlesy shalés; a black loam ; and basalts, usu- 
ally ared ochreous’ soil. | 
The end of the’ decomposition of rocks seems to be the 
production of 4'bed for vegetation. As‘ soon as the rock 
begins to be softened, ‘the seeds of lichens, which are con- 
stantly floating in’ the air, make it their resting-place. 
Their generations ‘Occupy it till a finely-divided earth is 
formed, which becdmes capable ‘of stipporting mosses and 
heath : a¢ted upon’ by igh heat, ‘these plants drink in 
the dew, and convert ddiistitdent parts of the air into nou- 
rishment. © Their dath aid decay ‘afford food for a more 
perfect species of verrelable!;: avid at‘ Tength a mould 1s 
formed, in which éven the ‘trees’of the forest can fix their 
roots, and which is capable of rewarding the labours of the 
cultivator. 
The decomposition of rocks tends to the renovation of 
Gg2 svilsy 
