The Small Rock-Garden. 139 
An occasional top-dressing with silver sand, in spring 
or autumn or both, is very helpful in keeping the 
surface soil of the rockery open and sweet. 
The kind of soil used for a rockery and the nature 
of the subsoil underneath have the greatest influence 
upon the aspect under which different species will 
thrive upon it; thus in a district with a retentive 
loam, a marly subsoil, and free drainage, and hence 
where the soil is liable to suffer from drought and 
to become baked in summer, many plants which 
usually do best in full sun facing due south can only 
be grown satisfactorily and with ease when planted 
on northern slopes or among stones with a northern 
aspect, though they must then be fully exposed to 
the early morning sun, and entirely protected from 
the winds and cold of the north. 
The relation between calcareous and non-calcareous 
geological formations, and the welfare of plants 
grown upon them, is also chiefly dependent upon 
physical considerations, subject to intelligent pro- 
vision and appropriate adjustment ; and even when 
this is not the case there are few instances in which 
the need of specific chemical elements peculiar to 
the one class of formation or the other cannot be 
satisfied by the incorporation of that particular 
material into the alien soil. Plants which are rather 
“tender” or only partly hardy, and which therefore 
require to be parched in summer in order that their 
growth may be firm and their wood become hard 
and well ripened to face an English winter and to 
ensure the next year’s flower, and which must besides 
be as dry as possible in winter, have their needs 
