PRESIDENTIAL ADDRESS. 23 



that the rocks amongst which they are growing, contain 

 upwards of 70°/° silica, much of which is in a free state, 

 while E. viminalis, nova-anglica, and stellulata, growing 

 perhaps only a few hundred yards away, usually indicate 

 that there is less than that amount of silica in the form- 

 ations which sustain their growth. But in dealing with 

 the acidity and alkalinity of soils produced from the 

 same type of rock in various localities, the question of 

 the local rainfall and topography must be considered, 

 for it will be at once apparent that a decomposing alkaline 

 rock in a dry climate will furnish a soil containing a higher 

 percentage of alkali than will be produced by a similar rock 

 in a wet climate, for the reason that in the wet area the 

 soluble alkali will be more readily leached out, and some 

 of it washed away. The difference in the alkalinity of the 

 resultant soils will be accentuated if the moist area is in 

 mountainous country and the dry locality on the level plains, 

 for in the former case the alkali is readily washed away 

 after leaching out, and in the latter, both operations are 

 retarded. The question however, is one that has to be 

 considered with caution in view of other factors which 

 may sometimes operate. 



In order to ascertain what are the various constituents 

 and conditions selected by certain plants, a botanical 

 survey should be carried out in conjunction with a soil 

 survey, and analyses made of the soil and rocks which 

 furnish the nourishment for the particular species under 

 investigation. Details should also be obtained of the 

 climate, rainfall and aspect of the area surveyed. When 

 this is done for New South Wales, we shall know why 

 distinct preferences are so often shown for particular 

 geological formations. 



It seems not unreasonable to suppose that the two 

 extreme types of rock, acid and basic, which furnish very 



