MAKING THE ROGK GARDEN 17 



of stone that may be used for the purpose in view. The 

 least satisfactory are those of flat, slab-like character. 

 It is very difficult to arrange these to produce a natural 

 rock garden, though they are less objectionable when 

 buried deeply. Such stones are, of course, useful for 

 placing beneath the surface, and for building walls, 

 but generally rocks of irregular shape and bold outline 

 alone should be chosen. 



One of the most expensive of all rocks is tufa, though 

 it is undoubtedly most attractive, and very useful. It 

 is useful because some alpines that are difficult to grow 

 have an especial fondness for tufa, and will grow there 

 while languishing in other stone. One may drill holes 

 in tufa, fill them with soil, and put in little plants with 

 every prospect of success, for the roots will penetrate 

 the rock, and there wax strong and lusty. 



The Geology of Rocks. The scientist has swamped 

 geology in a flood of technicalities, but it is quite 

 sufficient for the owner of a garden to divide all rocks 

 into two great classes stratified and unstratified. The 

 former, or sedimentary rocks, as they are called, are 

 well represented by shale, sandstone, and limestone, 

 in which layers or strata are distinctly noticeable; 

 the strata are considered to have been formed by the 

 action of water ; they are fossiliferous, and, what chiefly 

 concerns the gardener, are generally found in a more or less 

 regular condition in the natural state. Unstratified rocks 

 are igneous or volcanic in origin ; it is presumed that 

 they existed at one time in a molten condition, and now 



