BY E. O. MARKS, B.A., B.E. 101' 



If now we were to suppose that island to be composed 

 of a rock, uniform in texture and so exceedingly hard, 

 physically, but so readily decomposable that the chemical 

 action alone would be worthy of consideration, what different 

 land forms would result as compared with the usual hypo- 

 thetical developments ! 



Of course no rock known possesses these qualities 

 in such extreme proportions, but different rocks do differ 

 very greatly in their relative powers of resistance to 

 weathering and to friction. The ratio exhibited between 

 the resistance offered to these two destructive forces may 

 be termed the weathering factor of the rock, a factor which 

 would necessarily depend on the chemical or mineralogical 

 composition and on the texture of the rock and would vary 

 with conditions of climate. 



Some rocks, such as basalt or some coarse-grained 

 granite, being exceedingly hard and resistant to mechanical 

 action, are yet readily subject to disintegration by chemical 

 means. It seems to the writer improbable that the forces 

 of denudation would be capable under ordinary conditions 

 of inducing anything but rounded forms of a " mature " 

 aspect in large areas of such rocks. 



We have, for instance, in North Queensland extensive 

 basalt tablelands which have been dissected by narrow 

 gorges. These gorges have been cut in the arenaceous 

 strata below the basalt and are very narrow with precipitous 

 sides, while the watercourses on the surface of the basalt 

 are mere shallow depressions almost to the edge of the 

 gorge. Residuals of basaltic lava flows of greater age 

 occur both in North and South Queensland, and possess, even 

 where they have evidently been subjected to denudation i 

 for long periods, gentle contours on their summits from > 

 which the streams descend in waterfalls, being incapable • 

 of cutting back gorges or narrow valleys in such rocks. 



In the same way the undulating character of part of 

 the Burdekin Valley may be attributed to the weathering 

 factor of its component . granite, and not necessarily to a 

 maturity which it may, but quite likely does not, possess. 

 Of course such a granite, if on the edge of a plateau where 

 streams are sufficiently swift and powerful to make light- 

 of the natural hardness of the rock, would be excavated I 



