I] INTRODUCTION 37 



It will be seen that, whilst undertaken with* a different 

 object, the results of the observations on the upper atmosphere 

 by Professor Schuster and his staff have several important 

 bearings on vegetation, though the significance of these cannot 

 be correctly appraised until the observations have been con- 

 tinued for a considerable number of years. Though admittedly 

 incomplete, they still help to furnish clues as to the different 

 conditions under which vegetation lives at the different altitudes. 



NOTE ON THE USE OP THE WORDS " ACIDIC " AND "BASIC." Geologists 



have long distinguished between "acidic" and "basic" igneous rocks. 

 The former are poor, the latter rich in soluble mineral salts. From 

 the standpoint of the distribution of vegetation, the amount of soluble 

 mineral salts in the soil is a fundamental matter. Acidic rocks, soils, 

 peats, and waters are those which contain only a small amount of soluble 

 mineral salts ; and basic rocks, soils, peats, and waters are those which 

 contain a large amount. In time, when more analyses have been made, 

 it may be possible to express this relationship in quantitative terms ; 

 but, in the present state of knowledge, the matter can only be expressed 

 in a general way. Acidic waters are neutral to acid in reaction, basic 

 waters neutral to alkaline. 



