STABLE AND MIGRATORY PLANT FORMATIONS 67 



The grass covering such bosses is then very green and dwarf, 

 but many of the dolerite bosses in Scotland still have an acid 

 humus covering, an inheritance from past climatic conditions. 

 Dolerites contain a large proportion of alumina, lime, iron, 

 and magnesia, and a high percentage of phosphoric acid, but 

 the alkali content is low. The weathered surface is con- 

 tinually leached of its lime, but the oxides of iron and 

 silicate of alumina form a highly ferruginous soil of an easily 

 deflated, powdery nature when dry. In these cases, in 

 addition to the effects of the drought and lime in promoting 

 rapid decay, the poverty in humus may be in part due to the 

 oxidising effects of the iron oxides on decaying organic 

 matter. The grassland of these dolerite bosses in Scotland 

 is described by Robert Smith in his " Botanical Survey of the 

 Edinburgh District " (30). The association resembles in some 

 respects that of the chalk downs, but is much poorer in 

 species. 



In very dry positions, soils consisting largely of lime and 

 iron would appear therefore to favour rapid eremacausis 

 rather than humification, but these matters await the result 

 of experiment, and a short recapitulation may be given to 

 enforce the point, that the relations of lime to vegetation vary 

 widely with the physiography and climate. 



1. In arid regions lime accumulates in the soil apart from 

 the nature of the district rocks. 



2. In humid climates lime tends to be leached from porous 

 surfaces and elevated soils and to gravitate in solution to 

 lower levels. 



3. In sufficiently warm, moist, aerated soils with neutral 

 humus, lime in solution induces fertility through its powers 

 of liberating plant food and its favourable influence on 

 bacterial growth. 



4. In cold, saturated soils its main action may be one of 

 neutralisation and precipitation of soluble humus substances, 

 giving freedom of action to inorganic salts of ammonia and 

 potash. 



5. In humid climates, warm or cold, hard-jointed lime- 

 stone tends to develop an underground system of drainage 

 which may gradually lead to drought and eventually to the 

 barren condition know as limestone pavement. 



