300 GEOLOGICAL SURVEY OF NEW JERSEY. 
is claimed by many that forests do better on soils which have 
been cultivated for atime. The soil is mellowed by the process 
of cultivation, and the inorganic materials which have leached 
through the surface during the process of cultivation are reached 
by the roots of the trees. Pines which spring up in old 
abandoned fields are very vigorous, and in Virginia signs of the 
old corn-rows may be seen in forests on land which produced 
cotton and corn up to the time of the Civil War. At the same 
time the materials which have escaped the field crop by leaching 
are brought again to the surface by the tree so that the soil is 
being rendered fit again for agricultural purposes. The forest 
is thus an important factor in the rotation of crops, as I have 
already explained in the first chapter in connection with the 
eastern shore of Virginia. Although the farmers in that region 
remove the litter, they wisely allow old fields to come again in 
pines and clear fresh pine forest. By making the rotation of 
the pine short, the growing of pines in the sand-lands of the 
South may in time play an important agricultural role, and may, 
as one in the series of several crops, be necessary to maintain 
the productiveness of the soil. 
It must not be forgotten also that the roots of trees corrode 
even the silicates, and coarse sandy soil, under the influence of 
vegetation, becomes gradually loam-like in nature, although 
clay may be lacking. For this purpose pines have great advan- 
tages in that they are adapted to sandy soils and that the stumps 
soon rot after the tree is cut. It is not a difficult task to clear 
a pine forest for agricultural purposes. In planting old fields 
the surface should be plowed under in order to bring fresh soil 
to the top. The young crop demands a rich surface-soil ; later, 
when the roots penetrate into the subsoil upon which the tree 
feeds, it can take care of itself. Smets recommends, therefore, 
the growing of young plants in nurseries in good soil first, and 
claims that the so-called hardening of plants by growing them 
under adverse conditions is a mistake. This is, of course, expen- 
sive, and is not necessary in a country where a good crop is 
usually produced naturally or may be easily produced by sowing. 
There is formed in the heathlands of the Campine a peculiar 
powdery dry light-brown or black humus which decomposes 
with difficulty and collects in thick beds. It is formed mainly 
