314 THE DESERT SANDSTONE. 



The included fragments can sometimes be recognised, and if I 

 am not mistaken, small fragments of augite, labradorite and other 

 volcanic crystals, are amongst them. If this were beyond question 

 it would go far towards proving a volcanic origin for the sands. 

 Fragments of biotite, olivine, and other crystals associated usually 

 with igneous rocks have been apparently present, but in so small 

 a quantity and in such a fragmentary way that the evidence is 

 not conclusive. 



Finally there are the cavities in the quartz grains which seem 

 to me after having examined many specimens, to have something 

 peculiar and characteristic about them. Those who have not had 

 much experience in the microscopic examination of quartzose sands 

 can scarcely form any idea of the extent to which the grains are 

 full of cavities. There is no such thing as solidity in this mineral; 

 it is honeycombed to such an extent with minute bubbles, that no 

 fragment however small is free from them. They assume the 

 most fantastic shapes, not always rounded or oval like bubbles 

 generally, but compressed, flattened, twisted and spread out in 

 every conceivable form. Sometimes a succession of parallel lines 

 of cavities in one direction is crossed at varying angles by similar 

 lines, so as to give a clouded appearance to the grain. High 

 magnifying powers are required for the perception of a large 

 proportion, and each increase in the power of the objective brings 

 into view cavities whose existence was not previously suspected. 

 There does not seem to be much difference in this respect, between 

 the quartz of granites, volcanic sands, and crystals that have been 

 formed by slow infiltration ' without heat or pressure. At Mount 

 Bramble near Springsure, in Queensland, there is an extinct crater 

 on the volcanic tableland, the lava of which is covered with an 

 infiltration of hyalite, no doubt a slow result of weathering ; yet 

 the quartz is as full of cavities as the quartz grains from the ash 

 deposit of Mount Bromo, in Java. In the sandstone from the 

 Victoria River, which is an aggregation of purely siliceous grains, 

 in fact a quartzite, there is little else besides these cavities visible 

 in the transparent particles ; though even here small grains of 

 magnetite and other minerals are present, including particles of 

 brown augite, which are being converted into grass green mineral, 

 probably viridite. The sands of this, rock did not afford me a 

 sufficient number of examples to enable me to speak positively ; 

 but from what I have seen I think that the volcanic cavity is more 

 obliterated in this rock than in any other of the same character. 



It is not however impossible to recognize recent volcanic particles 

 of quartz by certain frothy aggregations of bubbles, which are 

 unmistakably indicative not only of former melting, but boiling. 

 Sometimes this gives rise to a ribboned structure as if the bubbles 

 had been drawn out by flowing. There are also roughly parallel 



