AGRICULTURAL GEOLOGY. 
125 
AGRICULTURAL GEOLOGY— No. 2. 
Clays being derived from the wearing down 
of rocks, must of course contain all the ingredi- 
ents and partake of the characters which those 
rocks possess. Thus the depth of a soil depends 
upon the rock being easily decomposed, and its 
texture depends upon the nature of the compo- 
nents. The number of chemical substances 
which soils derive from rocks are fourteen, 
namely : — 
Silica, or flint earth, Common salt, 
Alumina, or clay earth, Potassium, 
Lime, Sulphur, 
Potoxide of iron, Sulphuric acid, 
Peroxide of iron, Phosphoric acid, 
Oxide of manganese, Fluoric acid, 
Magnesia, Carbonic acid. 
Of these, the first three generally exist in the 
greatest amount, but the last seven are really 
the chemical elements of the soil. The oxide of 
manganese and protoxide of iron exist in few 
soils, and then only in small quantities. There 
are few rocks, however, which do not supply 
all of these substances in variable proportions. 
Granite rocks, which apparently furnish so 
little of soil, are yet the source of all the ingredi- 
ents found in soils. They are the rocks of the 
oldest geological period, and those found at a 
more recent date derive their materials from the 
primary granite. A knowledge of the deposi- 
tion and nature of these rocks is of indispensable 
necessity. 
This rock is called the basis or crust of our 
globe, from being that found lowest down, under- 
neath every other in position, if we except green- 
stone trap ; and if it appear at any time as a sur- 
face rock, or form those rugged hills which are 
scattered over Massachusetts, Maine, Vermont, 
and the noi'theast part of this state, it is because 
stratified rocks, o*r those of more recent forma- 
tion are wanting. It is termed an igneous or 
fire-born rock for two reasons ; first, because 
wherever found, the land is thrown into unu- 
sual elevations, having wild and sublime hills 
with pointed peaks, very unlike the round un- 
dulations of a limestone country, which hills 
have been raised by a gradual volcanic upheaval 
of the crust. The second reason is its crystal- 
line character. This is due to its having been 
at one period melted by subterranean heat, and 
cooling slowly under great pressure. 
Granite is generally composed of three min- 
erals, not always found in the same proportions, 
the variations giving the peculiarity to the rock 
of the locality. These minerals are quartz, 
felspar, and mica. They are themselves com- 
pound substances grouped together in definite 
proportions, amounting to eleven in number and 
are arranged to form the following bodies : — 
Silica, Potash, Fluoric acid, 
Alumina, Soda, Phosphoric acid, 
Lime, Iron, Water. 
Magnesia, Manganese, 
The quartz, which forms the crystalline part 
of granite, is composed solely of silica. When 
it exists in any amount, it gives a hardness to 
the granite, and a want of fertility to the soil. 
Felspar is the pale white or reddish mineral, 
the most important in an agricultural point of 
view, both because it decomposes readily, and 
because it furnishes the valuable food of plants. 
Its ready decomposition is due to the large 
quantity of potash which it contains. It yields 
five elements to the soil by its wearing away 
and is composed in 100 parts of 
Silica, 68.0 
Alumina, 20.0 
Potash, 8.3 
Lime, 2.0 
Oxide of iron, 0.5 
.Loss, 1.2 
100.0 
Some varieties of felspar contain as much as 17 
per cent, of potash. Leaving out the lime and 
iron, which are in small amount, silica, alumina 
and potash are the chief constituents. The first 
is an acid, and tends to unite with alumina and 
potash, forming a silicate of alumina, and a sil- 
icate of potassium. In this state in the rock, they 
are insoluble, but when the latter becomes ex- 
posed to the air and weather, this arrangement 
is broken up, the carbonic acid of the air being 
the chief cause ; this acid unites with the potas- 
sium to form carbonate of potash, and the silica 
is displaced as free sand, so that a decomposed 
felspar is thus made up of a silicate of alumina, 
carbonate of potassium, and silica. 
Silicate of alumina is fine pottery clay, and is 
that which gives cohesion and the retentive 
property to soils ; the carbonate of potash has 
all the properties possessed by common pearl- 
ash with which it is identical. In fact, all the pot 
and pearl ashes of commerce are derived from 
felspar rocks. This potash dissolves readily in 
water and is carried away from the side of the 
decaying rock into the soil below, where it is 
distributed through the ground by the currents 
of water constantly flowing through the soil. 
The more felspar a granite contains, the more 
valuable it is, inasmuch as it forms a deeper 
soil, it yields more alumina, or clay, which gives 
cohesiveness to the soil, prevents the soluble 
saline matters being washed out, and contrib- 
utes large quantities of carbonate of potassium, 
so useful a mineral to many plants. The abso- 
lute quantity which felspar can yield is much 
greater than could be supposed at first view, 
one Hessian acre, or 40,000 square feet, twenty 
inches deep, being able to afford 1,152,000 lbs. 
of potash. Hence it is that attempts have been 
lately made, and with success, to manufacture 
the commercial potashes out of felspar. It is 
from this abundance of potash that such crops 
as flax, and potatoes grow so well in granite 
soils, these two plants requiring potash as an 
essential element 
Granitic districts, not elevated much above 
the sea, are naturally fertile, for they contain 
the fine clayey matters which have been washed 
down from higher districts, and the warmth of 
the sun is greater. In fact, it is the high and 
exposed situation which granitic countries gen- 
erally occupy, that renders them so barren. 
This is the case with the district between Lakes 
Champlain and Ontario, the granite region of 
