BY R. H. CAMliAGK. 313 



It is worthy of note that in the surrounding districts, where 

 the formation is sedimentary, granitic, or gneissoid, fair forest 

 trees are found, though, owing to the exposed situation ^<nd only 

 medium rainfall, the timbers are not of the finest. Even under 

 present circumstances, had the site of Monaro Plains been com- 

 posed of siliceous rocks instead of basic, it is not improbable that 

 it would have been fairly well covered with forest trees. 



Some of the reasons which account for this tableland being so 

 thinly timbered may be briefly stated as follows : — the rainfall 

 being only moderate, the rock-formation being basic rather than 

 siliceous, absence of shelter, the great degree of cold, and the 

 dryness of the prevailing summer winds which come from the 

 west and north-west. 



A study of the following rock-analyses in ver}' instructive. 

 Contrast the low percentage of silica in the basalt, with 

 the high percentage in the gneiss. A gtanite containing 

 less than 65 to 70% of silica is not remarkably acid, but when 

 that amount is exceeded, the rock becomes decidedly siliceous. 

 It is interesting to observe the gradation in the amount of silica 

 from the basalt to the granites, quartz-porphyry and gneiss; and 

 then to compare this with the increased forest -growths on the 

 latter rocks over tlie first-named. If this result is actually due 

 to the greater quantity of silica, it surely must be in some degree 

 owing to the physical properties of the siliceous soil, rather than 

 to the chemical constituents, for pure quartz cannot he regarded 

 as a nourishing plant-food. Possibl}^ the capillarity of the 

 particles in the siliceous soil is greater than that of the basaltic, 

 and the former is, therefore, better enabled to re^^ain mo'stureand 

 theieby sustain larger growths. The basaltic soil, in view of the 

 limited rainfall, is suited only for the periodical production of 

 smaller plants in response to damp seasons. 



It maybe noticed tliat, as the amount of silica increases in the 

 rocks, various other constituents which go to furnish a good soil, 

 gradually decrease.'^' The fact remains, however, that the basalt 



*At the same time, the quantity of potash increases, and may have some 

 considerable influence in producing the forest-growths. 

 31 



