414 Geology . 
Ing in tlieir tarn terra firma. The sea is propelled over 
the old continents, which then become the bottom of the 
ocean, while the new continents are gradually clothed 
with vegetables and animals, and in process of time un- 
dergo the same gradual action, decay, and submersion, 
which their predecessors experienced. 
The experiments of Sir James Hall, on the effects of heat 
modified by compression, have been made in pursuance 
and support of Hutton’s theory ; and he has certainly 
shewn, that crystallized forms of oarbonat of lime may be 
produced under the joint operation of great heat and great 
compression, which are very similar to such as would be 
generally ascribed to a crystallization from watery fusion 
or admixture. 
The view of the subject that at present occurs to me 
as most probable, is this : 
The density of the earth, according to the calculations 
and observations of Sir Isaac Newton, Dr. Maskelyne, 
Dr. Hutton, Professor Playfair, and the honourable Mr. 
Cavendish, cannot be less than five times that of water. 
The strata or formations of the earth, so far as they 
have been examined, consist of the following nine earths 
or their combinations : alumina, silica, calcia, magnesia, 
baryta, strontia, glucina, zirconia, yttria. The three last 
are found in quantities so comparatively small, as not to 
be worth notice on the present occasion. The same ob- 
servation may be made, though in a less degree, on ba- 
ryta and strontia, which are only found occasionally in 
secondary strata in nodules, or as the matrix of ores, or 
otherwise insulated. The earth and its formations may 
therefore be considered as consisting of alumina, silica, 
calcia, and magnesia, and tlieir admixtures and combi- 
nations ; interspersed rather than intersected occasionally 
by metallic substances. 
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