Royal Institution. 395 
magnesia, lime, fixed alkali, and oxide of iron. These 
substances, variously combined, give rise to the great va- 
riety of the forms and appearances of their crystals; and 
Mr. Davy’s late discoveries prove that the earths and alkalies 
have metallic bases, and that they are compounds of these 
bases and oxygen. 
In the arrangement of the primary rocks a certain order 
and relationship is generally to be observed—granite is the 
highest and deepest rock, it forms the summit of the loftiest 
mountains, and seems to be the foundation ofour conti- 
nents and islands, and is usually covered by geogneis (itself 
a species of granite), micaceous schist, or sienite. Ser- 
pentine and marble occupy the middle stations of mountain 
chains, and are oftener found upon micaceons schist than 
upon granite. 
Porphyry is mostly'associated) with ‘granitey'and is fre- 
quently’ immediately ‘iteumbent upon? it. © The primary 
rocks Constitute’ the®priticipal solid part of the surface of 
our globe, and form the'lottiest mountains, and their geo- 
graphical position is admirably adapted to preserve the or- 
der and ceconomy of the system—The heights of moun- 
tains in general diminish from the equator towards the 
poles. In hot climates the effects of mountains are to 
lower the temperature of the subjacent countries, and thus 
most of the tropical regions are rendered habitable. Where 
mountains do uot occur, as in Africa, there are sandy de- 
serts ;—mountains too are the sources of streams and rivers, 
they attract clouds, condense vapour into water, and pour 
it into the valleys and plains; they modify the course of 
the winds, and shelter the lowlands. : 
In all systems for explaining the formation of primary 
rocks, a fluid state either from igneous or aqueous fusion 
is assumed, but it is not yet explained. Mr. Davy remarked, 
‘why different crystals should separate in the same mass. 
The crystals produced either from solutions or by slow 
cooling in artificial or natural operations are uniform, and 
not of different species, like those of the primary rocks; so 
that the solution of this grand problem, if it be capable of 
solution, must be gained from the improvement of che- 
mical science. Nature may produce effects by powers 
which have not as yet been discovered, and her resources 
should never be estimated by our operations. 
Mr. Davy, in the course of this lecture, mentioned the 
applications of the substances derived from the different 
primary rocks, to the uses of common life, and particularly 
pointed out their applications to the purposes of architec- 
, i ture. 
