May, 1938 The Queensland Naturalist 
85 
instead a rough, somewhat pitted appearance. 
In addition to the above types, all of which are 
plentiful enough, one sees here and there pebbles of an 
exceptional kind. The more interesting of these rarities 
that I have noticed, is an angular fragment of chalcedony 
and a highly polished specimen of fossil wood. 
From the petrological point of view, we see that 
the pebbles display considerable variety, and it is a 
reasonable assumption that they originally had a wide 
distribution. But in spite of such patent diversity, the 
pebbles constituting the gravel have one all-important 
factor in common: they are all exceptionally durable. 
They have all undergone the ordeal by water and have 
emerged battered and worn but triumphant. 
The factors necessary to ensure that durability with- 
out which a pebble would soon cease to exist are hard- 
ness, or toughness, or insolubility, or some combination 
of these three. Any rock which is soft or easily broken 
or readily dissolved is unsuitable for pebble formation, 
and is abraded, broken, disintegrated or dissolved be- 
fore it has travelled very far along the bed of the stream. 
As it happens, the most insoluble of the common rock 
materials is silica (Si02). Not only that, but silica in 
its several mineral forms is also the hardest of such 
materials. It is to be expected, therefore, that siliceous 
pebbles of one kind and another should be prominent 
members of most gravels, and this expectation is height- 
ened by the knowledge that silica is the commonest 
material on the crust of the earth. 
We have here a simple explanation of the prepon- 
derance in the gravel path of such substances as cherts, 
jaspers, and quartzites, all of which are very highly 
siliceous. 
An explanation of the common presence of ande- 
sites is somewhat different. These rocks are not highly 
siliceous, but on the other hand they are so extremely 
tough that for many years andesites (or blue metal, as 
they were commonly called) have been selected by 
engineers for use in making macadam roads. The 
durability of these rocks is largely due to the toughness 
of the mineral hornblende, which is one of their chief 
constituents. The other mineral (plagioclase felspar) is 
not nearly so tough, and it is this difference that causes 
the andesitic pebbles to have a somewhat rough, pitted 
appearance, quite distinct from the uniform smoothness 
of the cherts. 
Having considered very briefly the chief character- 
