; 
y 
Kitroe—The Distribution of Drift in Ireland. 425 
regarding rocks, in the agricultural point of view, is their chemical 
composition, seeing it is from this that the elements of natural 
fertility must be drawn. 
Soil, earth, or “clay,” as known to the agriculturist, consists 
of a mixture of pure clay (hydrated silicate of alumina), usually 
with finely-divided felspar, or felspathic mud, undecayed rock 
particles, small stones, and more or less organic matter or humus 
(i.e. decayed vegetable and animal substances). If the stony 
particles were entirely withdrawn, and the impalpable mud or 
clay only were used for plant growth, it would soon become 
exhausted of whatever mineral nutrients it contained. The stony 
particles and fragments contain the stock supplies of fertilizing 
minerals. ‘hey are constantly undergoing decomposition in the 
field by oxidation and solution, owing to which their enrich- 
ing elements are transferred to the clayey matrix.1. An observant 
person, noting the stony contents of a soil, may form a fair 
judgment of the chemical constituents he may count upon in it 
—a course which will direct him, in the absence of chemical 
analyses, towards the most advantageous selection of manures. 
Here, also, we find the reason of the well-known advantages 
attaching to rotation cropping. Crop after crop of the same 
kind, which would draw most largely upon one or two minerals 
in the soil, would exhaust the storehouse of these minerals; 
while a rotation of crops gives time, before the same crop is 
repeated, for the replenishment of the storehouse at the expense 
of the stony particles. 
Soils derived severally from the rocks upon which they rest 
would rarely possess all the constituents essential to fertility. 
Those formed by the disintegration of acid igneous rocks, granites, 
felsites, &c., felspathic grits, sandstones, quartzite, shale, slate, 
and mica-schist would lack or be deficient in several important 
ingredients such as lime; while those derived from basic igneous 
rocks, basalt, dolerites, diorites, serpentines, &c., limestone, and 
calcareous grits will be deficient in potash. 
The celebrated fattening lands of Meath and North Kildare— 
1The matrix possesses the notable property of absorbing the leading manurial 
_ substances—phosphoric acid, potash, lime, magnesia, and soda, as well as ammonia. 
It therefore serves as a convenient storehouse wherein nutriment is temporarily laid up 
in available form for crops. 
2H2 
