ontrive plenty of long soil runs. Eighteen inches
should be the very least. A plant like the alpine androsace is a tiny
rosette, seemingly requiring no more than an inch or two of soil, but
its roots are likely to be found following an earth-filled crevice in
the rocks to the depth of a yard or so. It is because of this deep
penetration of roots that the soil should be packed so very firm; the
roots must be in no danger of loose soil or of striking a hidden
hollow.

[Illustration: Where a rock would bear too heavily on the one below it,
even with soil between, the pressure may be relieved by the use of small
stones. The soil run need not be straight, but it must be continuous, so
that the roots of the plant may find their way from A through to B]

At no point between two stones should the layer of soil be less than two
or three inches thick after being packed hard. If an upper stone is
likely to bear down too heavily and crush the plant roots, this may be
avoided by placing small stones here and there in the layer of soil. The
roots will work between these stones, but there must be a continuous,
though not necessarily straight, soil run from the front of the rock
work to the solid filling of earth. The run should slope downward
slightly.

Rocks calculated to simulate a natural stratification ought to be laid
on an incline for proper drainage. Such pieces of rock may also be
employed sparsely in wedging, and in the making of the so-called
"pockets."

These pockets are of prime importance in the construction of a rock
garden. They hold the only considerable spaces of soil and are the chief
means of colonizing plants, thus providing for pronounced color effects.
They should break the slopes and be irregular in size, shape, and
distribution. The large ones may be easily subdivided by small stones
when the planting is done if a further separation of species is
desirable. The soil must slope a little from the top, so that there will
be no standing water.

[Illustration: Cross-section of rock garden construction, showing
shallow (A) and deep (B) soil pockets; tilting and wedging of rocks (C);
bridging (D), and perpendicular crevice soil run (E). Two to three
inches of soil between all joints. The lowest rocks are partly buried]

The drainage of a rock garden is of vital importance. There must be
plenty of moisture stowed away behind the rocks against the heat of
summer, but all excess must be carried away. The garden sh