40 



Essential Structural Details 



Apart, too, from the sympathetic and nourishing effects 

 of natural rock, are the addled advantages of larger 

 cavities, greater grit and soil spaces, than would be 

 possible in brickwork. To the plants these have an 

 importance of their own. 



The Set-Baok Ledge.— In the process of building 

 up the planting face of the wall, three things are abso- 

 lutely essential. These are : (i) a slight dip or tilt of 

 each stone from front to back (see Fig. 20) to carry 

 water to the roots of the plants ; (2) the set-back ledge 



Fig. 19. — Elevation of a Dry Wall. 



The shaded parts represent soil ; the crosses indicate 

 positions for plants. 



or front of the stone so that the one immediately below 

 it intercepts rain or applied water; and, (3) that all 

 spaces or crevices between stones be filled up with soil 

 as the work proceeds. At the back of the stone work, 

 and between it and the concrete retaining wall, a soil 

 cavity not more than three inches wide might be 

 arranged, and if charged with pulverised rock, old ceil- 

 ing plaster, grit and loam in about equal parts, there 

 will be presented to the roots of plants, in modified 

 degree, the indeterminable fissure of the mountain, the 



