714 VARIABLE CHARACTER OF VEGETATION ON BASALT SOILS, 



Before proceeding to discuss general principles, I desire to 

 review the various types of vegetation which I have observed on 

 basalt-formations. They will be dealt with in the order of their 

 extent and importance. 



A. Basalt >S'cr2*6i,'(Jungle-forests or Brushes). — Wherever sheets 

 of basic lava cover large areas in the coastal districts of Queens- 

 land, or of New South Wales north of the Nambucca River, 

 magnificent "scrubs" are found to be coextensive with the basalt- 

 formation. Only here and there may a patch of forest-country 

 be seen, and on closer examination it will always be found that 

 such a patch is accounted for by either — 



(a) Rapid drainage of the soil, as on the summit of a peak or 

 of a razorback range; or 



{b) The existence on the spot of a formation of porous tuff; or 



(c) The })resence of a felspar-basalt, porphyritic in acid plagio- 

 clase, which, by resisting weathering longer than the other con- 

 stituents, gives a more sandy and porous texture to the soil. 



So that where a forest-patch is found on basic volcanic rock in 

 a tropical or subtropical coastal area, the occurrence is always due 

 to superior porosity and drainage of the soil. The forest-trees 

 which predominate on such patches are always those which are 

 typical of good soils on other formations, namely, box, ironbark, 

 apple, and blue-gum. 



In isolated places where a depression without an outlet occurs 

 in basalt country, either on a peak or on a plateau, an absolutely 

 treeless stretch of country may occur. The only vegetation con- 

 sists of reeds and brush. Such barren areas are the result of 

 complete absence of drainage, leading to the soil becoming sour, 

 and the soil-water becoming saturated with soluble salts delete- 

 rious to plant-life. 



^.Basalt Plains. — West of the Great Dividing Range, large 

 basalt-areas are almost treeless. The Darling Downs possess 

 only a few straggling trees on the small knobs or hillocks, which 

 are dotted over it at intervals. The hillocks invariably possess 

 a more stony and porous soil than the level country, hence the 

 trees can take root. In New South Wales, similar facts may be 



