INFLUENCE OP ROCKS UPON THE FERTILITY OP SOIL, &C. 39 
along the Cordillera of the Andes, and in Mexico, where it is 
rich in the ores of the precious metals. Some varieties are ex¬ 
tremely l)ard. Sucli was that whicli was quarried and wrought 
by the ancient Egyptians, and wliich being of a reddish or pur¬ 
ple colour, gave a name from the shell-lish 
yielding the purple dye) to the rocks having this constitution. 
We have some varieties of flinty slate in our transition forma¬ 
tion that are porphyritic, but the only rocks in North Carolina 
that liave any claim to belong to the class of which we are now 
treating, arc the greenstone dykes that traverse the old-red-sand¬ 
stone and the natural walls in Rowan. Connected with the old- 
rcd-sandstone formation there arc in the northern states immense 
masses, rising sometimes into considerable mountains, of basaltic 
greenstone, which are supposed by geologists to be of volcanic 
origin, though no one pretends to tell how they gained the position 
they now occupy. The same substance presents itself in the sand¬ 
stone of North Carolina, not swelling into such large masses, but 
forming dykes of a few yards in breadth or hills of moderate ele¬ 
vation. 
The natural walls in Rowan have been several times described. 
A French naturalist, well known in the scientific world by an 
able work on tbe insects of Africa, and another on a family of 
cryptogamous plants, (M. Palisot de Beauvois) passing from the 
country of the Creeks to Pennsylvania, beaial at Salisbury of tbe 
natural wall, and wishing to know more of so singular an object, vi¬ 
sited it and carried specimensof it to Paris. These were put into the 
hands of the alilest French mineralogists, M. M. Broignart, Bro- 
chant, and Gilet Laumont, who all decided that these stones had 
at least all the characters of basalt, tbough they hesitated about 
pronouncing them to be that substance by reason of tbeir being 
found in tbe heart of a ]irimitive country. Such walls or d 3 'kes 
are met with, though not in the same geological connexion, in 
other parts of the world. There is one that traverses the coal 
fields in the north of England for manj" miles. But McCulloch 
remarks respecting one variety of granite that it is undistinguish- 
able from laasalt, though it is connected with and passes into gra¬ 
nite of the most common character. ^Vhether the natural walls of 
Rowan are basaltic dykes or contemporaneous granite veins, there 
is perhaps room for doubt. 
INFLUENCE OF THE ROCKS UPON THE FERTILITY 
OF THE SOILS PRODUCED BY THEM. 
24. It is in connexion with the art and science of agriculture 
that the strata of the earth exhil)it their most interesting relations. 
IMan depends for his existence upon the power possessed by vege¬ 
tables of attracting the elements (carbon, oxygen, hydrogen and 
nitrogen) of which thej' are constituted, combining them so as to 
form vegetable matter, and incorporating them with their own sub- 
