214 GEOLOGICAL SURVEY 
crop of corn, growing on the sides of a hill covered with the crumbling ruins of the rocks that had 
fallen from the top; the soil seemed to be composed of pulverised granite. 
I believe that the comparative excellence of the soils derived from the granitic and metamorphic 
rocks of the State, is mainly due to the fact, that while the disintegration of the rocks proceeds 
rapidly and to an extraordinary depth, the decomposition of their mineral constituents advances but 
slowly, so that when, through injudicious culture or other cause, the soil is washed away by the 
rains, instead of a bare and hard rock, there is left a subsoil composed of its crumbling constituents, 
wanting but the addition of vegetable or other organic matter to form again a productive soil. 
Although I have taken granite as an illustration of the connexion between rocks and soils, what 
has been said of it will hold true of the other rocks, which are, for the most part, made up of the 
same minerals, in various proportions and combinations, and, of course, affecting the soils derived 
from them to the same extent. When quartz abounds in the rocks, the soil will be light and sandy, 
but when felspar and mica predominate, the soil will be stiff and clayey. Where mica prevails the 
soil is generally red, on account of the large quantity of iron in that mineral. 'The terms, red and 
grey, applied to granitic soils in the State, only indicate the proportion of iron present, and are no 
further useful as the basis of classification. : 
Having said thus much on the origin of soils, we may proceed to the further consideration of 
their composition and mode of action. 
A soil answers a triple purpose in relation to plants. 1. It serves as a support or foundation in 
which they may extend their roots. 
2. It furnishes a supply of inorganic elements for their food. 
3. It serves as a reservoir for water, and for organic matter which is taken up, in a state of solu- 
tion, together with inorganic substances, by the plants. 
In the greater number of soils, the inorganic matter or mineral constituents greatly predominate, 
constituting, generally, 95 per cent. of the entire mass. A soil, however, to be at all productive, 
must not fall below 1 per cent. of organic matter, and in our swamps it often amounts to 60 
per cent. 
Oats and Rye will grow on land containing 1 per cent. of organic matter; Barley requires not 
less than 3 per cent; and Wheat will not flourish in a soil containing less than from 4 to 8 per cent. 
In all soils the predominant inorganic constituents are Silica, Alumina and Lime. Silica is well 
known in the common form of sand; alunaina is the basis of clay; and lime is too common a sub- 
stance to need description. 
Silica exists in the soil, in part, inan uncombined state as sand, which may be separated by simple 
washing, and combined with alumina as a silicate. As one or the other of these predominates, the 
soil is denominated sandy, argillaceous or calcareous. A soil may be denominated calcareous when 
it contains 20 per cent. of lime. 
A good classification of soils, though highly desirable, obviously presents many difficulties. 
Taking the two leading ingredients, silica and alumina, as the extremes of the scale, a very simple 
classification may be formed.* 
1. Pure clay, composed of 60 parts silica and 40 alumina, in chemical combinationt—no siliceous 
sand. 
* Johnson. 
+ Such a clay is a silicate of alumina, and not a mixture. 
