OF NATURE. 
53 
it was cut tranfverfly by a vein of clay, was 
changed into a pure filver . The number there- 
fore of fpecies, and varieties of foffils, each 
ferving for different purpofes, according to their 
different natures, will be in proportion, as the 
different kinds of earths and Jiones are vari- 
oufly combined. 
§• 4 * 
Prefervation. 
As fojfils are deftitute of life, and organisa- 
tion, are hard, and not obnoxious to putre- 
faction ; fo they laft longer, than any other 
kind of bodies. How far the air contributes 
to this duration it is eafy to perceive, fince air 
hardens many Jiones upon the fuperficies of the 
earth, and makes them more folid, compact, 
and able to refit the injuries of time. Thus 
it is known from vulgar obfervation that lime y 
that has been long expofed to the air, becomes 
hardened. The chalky marl , which they ufe 
in Flanders for building houfes, as long as it 
continues in the quarry, is friable > but when 
dug up and expofed to the air, it grows gra- 
dually harder. In the fame way our old walls, 
and towers gain a firmnefs in procefs of time, 
E 3 and 
