( (360) 
ly diffused, and form the crust of our globe. These 
are silica, alumina, lime, and magnesia. The first 
is the basis of quartz, sand and gravel; the second 
of clay; the third, of bones, river and marine shells, 
alabaster, marble, limestone and chalk; and the 
fourth, of that medicinal article known by the name 
of calcined magnesia. Ina pure or insolated state,(1) 
these earths are wholly unproductive ; but when de- 
composed and mixed,/ 2) and to this mixture is ad- 
ded the residuum of dead animal or vegetable mat- 
ter,’ 3) they become fertile, take the general name 
of soils, and are again specially denominated after 
the earth that most abounds in their compositions 
respectively. Hf this be silica, they are called sandy ; 
if alumina, argiliaceous ; if lime, calcarious ; and if 
magnesia, magnesian. ‘Their properties are well 
known: a sandy soil is loose, easily moved, little 
retentive of moisture, and subject to extreme dry- 
ness ; an argillaceous soil is hard and compact when 
dry, tough and paste like when wet, greedy and te- 
(1) See Gisbert’s experiments on pure earths and their mixtures. See also Da- 
vy’s Elements, p. 156. 
(2) In ‘this respect nature has been neither negligent nor niggardly, if (as 
Fourcroy asserts) the purest sand be a mixture of quartz, alumina, and sometimes of 
calcarious matter. Speculative geology is romance, and does not merit the name of 
Science ; yet is science obliged to borrow her theory of soils. 
and cold, moisture and dryness, decomposed the mountai s of tive, secondary 
and tertiary formation ; rains and the laws of gravity, brought these broken parts 
from places of more, to those of less elevation—where, by Thestieaiie) mixture and 
chemical combination, the present substrata were formed. But these were yet na- 
ked and unproductive, when the Cryptogamia family (mosses and lichens) took pos- 
. session of them, and in due time produced that vegetable matter, which made the 
earth productive and the globe habitable ! . 
* (3) Dead animal and vegetable matter, in the last stage of decomposition, give a 
black or brown powder, which the French chemists call ferreau, or humus, and 
which Mr. Davy calls an eatractive matter; this is the fertilizing principle of spits 
and manures. 
