716 VARIABLE CHARACTER OF VEGETATIOX ON BASALT SOILS, 



soil, though somewhat richer in mineral plant-food. The Blackall 

 Range Scrubs, in South Queensland, are of this type. The soil 

 is chemically very rich, and consists of dark brown or black 

 clayey loams. It has high water-retaining power, but low 

 porosity; and always suffers from sourness, and lack of aeration. 

 This is, however, no disadvantage to the typical scrub-plants, 

 which will flourish only in sour, heavy, and wet soils. On steep 

 slopes, the soils of basaltic scrubs may, through leaching, be 

 poorer than usual in this type of country. 



B. Basalt Flams. — The soil of these areas has the same charac- 

 teristics as that of the scrubs, namel}^ depth, richness in plant- 

 food, high water-capacity, low capillary power, and lack of 

 aeration. The cause which produces the dearth of vegetation is, 

 that those Australian trees which can live in heavy, impervious, 

 unaerated soils, namely the scrub-flora, are prevented from estab- 

 lishing themselves by the lack of rainfall, and sometimes by the 

 cold climate of the tableland as well. It is more difficult to see why 

 the forest-vegetation of more acid formations has been unable to 

 adapt itself to the much richer soil of the basalts. If the typical 

 forest-flora of Australia is very old, as we have reason to believe, 

 it is reasonable to suppose that it possesses an hereditary aversion 

 to soils of a heavy, clayey nature. 



Prior to the Miocene, there was little basic rock in Eastern 

 Australia, and the greatest basalt-areas are still later, namely, 

 Pliocene. Before the great basaltic extravasations, almost all 

 the soils of this continent were of a loose, porous, sandy nature. 

 Such soils, though poor in plant-food, are not only well aerated, 

 but are also able to supply any deBciency in rainfall by absorbing 

 moisture from the dew or from the atmosphere by capillarity. 

 If heavy rains fall, the surplus water readily drains away. But 

 the basaltic soils, of the Miocene and Pliocene outpourings, are 

 heavy, clayey, impervious to air and water. They have so high 

 a water-capacity, and so poor a porosity, that in heavy rains they 

 become water-logged, and the tree-roots are suflfocated; while in 

 a prolonged drought they dry up to such an extent that the trees 

 die of thirst, the capillary power of the soil being so slight that 



