PRESIDENTIAL ADDRESS. 21 



Geology. — The effect of geological formations upon the 

 distribution of plants, though distinctly evident in many 

 localities, is to some extent of a local nature, being domin- 

 ated by the influence of climate. Broadly speaking the 

 Eucalypts distribute themselves under two extreme types 

 of geological formations, the siliceous and the basic, and 

 there are numerous examples of two distinct floras 

 approaching each other up to a common boundary without 

 intermingling, the one growing perhaps on an acid granite 

 or siliceous sandstone formation with an abundance of free 

 silica, and the other on a basalt or other basic rock, pro- 

 ducing a clay soil. 



Although certain trees show such a distinct preference 

 for either a basic or a siliceous formation, it is difficult to 

 ascertain the exact reason for this discrimination. In 

 studying the analyses of a series of rocks, it is noticed that 

 broadly speaking, the ferro-magnesian constituents increase 

 as the silica decreases, and the basic rock therefore pro- 

 duces the more chemically-rich soil; while on the other 

 hand those granites with a high percentage of silica are 

 usually richer in potash than are the basic rocks. The 

 proportion of soda, however, has no regular gradation 

 according to either increasing or decreasing quantities of 

 silica, but of the two types of rock, the soda usually occurs 

 more plentifully in the basic than in the highly siliceous. 

 Apart from the question of the chemical constituents there 

 is that of the mechanical condition of a soil, and it seems 

 undoubted that in certain cases the physical characters 

 are of greater importance to the plant than the chemical 

 constituents. It is of course those rocks with a high 

 percentage of free silica which so largely affect the physical 

 characters of the soil, and furnish it with capillary pro- 

 perties, so that when a plant exhibits a definite preference 

 for a highly siliceous formation it may be that it neither 

 wishes to avoid some constituent which occurs in a large 



