INFLUENCE OF ROCKS UPON THE FERTILITY OF SOIL, &C 41 
Of these four concurrent causes of productiveness or sterility., 
the last has little connexion with the science of Geology. The 
influence of the others will be stated in accordance with what has 
been observed in other countries, and is laid down in the books ; 
after this we may enquire how far these representations agree 
with the facts that may be observed in regard to the soil of 
North Carolina. 
25. Composition. The simple minerals constituting the crust 
of the globe are composed almost exclusively of four kinds of 
earth ; silica, alumina, lime and magnesia, combined with each 
other in different proportions. It is therefore of these four earths 
that soils are formed in every part of the world. They contain 
besides, a little iron, and a minute quantity of the rarer earths and 
metallic oxides, but these are not abundant enough to affect in 
any considerable degree their fertility. 
So far is magnesia from increasing the fertility of the soil, that 
it may be doubted whether its influence is not positively injurious 
to all the forms of vegetable life. Thus the occurrence of soap- 
stone rocks, which owe their peculiar character to the presence of 
this earth, is always marked by sterility and barrenness in their 
neighbourhood. Of the other earths, no one will by itself form 
a good soil. Vegetables will not grow in a pure siliceous sand, 
whether coarse or fine. Some require a larger admixture of alu- 
mina than others. The long leaved pine (pinus australis of Mi- 
chaux) will thrive where corn will not grow ; but even this be- 
comes dwarfish, if the sand predominate so as nearly to exclude 
the clay. If on the other hand the alumina pass a certain limit, 
a clay so close and dense is formed, and so liable to change with 
every change of the weather, acquiring almost the hardness of a 
baked brick in a dry season, and becoming so overcharged with 
moisture in a wet one, that vegetables will not thrive in it. The 
chalk formation in England and France furnishes evidence that 
limestone does not by itself produce a good soil. 
It is by a proper mixture of these earths and of a certain quan- 
tity of decayed vegetable matter, all in a state of minute division, 
that a soil of the highest possible degree of fertility is created. 
Silica and alumina combined in proper proportions, will of them- 
selves form good land, but the addition of a little lime increases 
greatly its productiveness. Hence arises the fertility of some 
of the western states. The counties about Lexington in Kentucky, 
are not surpassed in this respect (some river bottoms excepted) 
by any part of the world, nor resting as they do upon a founda- 
tion of limestone, is there reason to apprehend that they will be 
exhausted by constant tillage. The influence of a quantity of lime 
upon the productiveness of a soil, is well exhibited in two small 
tracts, in the low-country of North Carolina ; the Rich-lands of 
Onslow, and that bearing the name of Rocky-point, on the north- 
east branch of the Cape Fear, in New Hanover. A bed of shell 
limestone, which underlies this part of the state, here rises to the 
