316 THE DESERT SANDSTONE. 



in geology in Europe, therefore we must not be surprised or 

 disappointed at the same thing happening here. It is the Desert 

 Sandstone which is being dealt with now, but it will be obvious 

 to any one who has paid even a slight attention to the subject 

 that this is applicable to some portions of the Hawkesbury 

 Sandstone as well. A considerable thickness of the upper strata 

 is composed of tufa or Tertiary ash-beds. This is especially 

 applicable to some of the Sydney sands and sandstone and the 

 strata on the Nepean River. 



It will be remarked also that the form of these ash-deposits is 

 nearly always crescentic with reference to the volcanic rocks, and 

 that the thickest portion of the beds and the greatest extent is 

 exactly in keeping with what we might expect as the effect of 

 prevailing winds. Many instances of this can be seen on all 

 geological maps where a survey has been made. 



For those who are not familiar with volcanic phenomena it 

 would be hard to realise that a mass of sandstone is nothing more 

 or less than an accumulation of volcanic ashes. The word ash 

 does not represent ashes in the ordinary acceptation of the term. 

 We must remember what a volcano is. We speak of smoke and 

 flame, ashes and cinders in connection with volcanic eruptions ; 

 but there is no such thing as smoke, as the word is usually 

 understood, and no such thing as name, unless sulphurous fumes 

 •can be called such. The smoke is steam intermingled with 

 quantities of finely divided stone fresh from the melting cauldron, 

 but blown into the finest particles by incessant explosion. The flame 

 is the reflection on the clouds of steam of the incandescent molten 

 rock rising from the depths of the earth. The ejectamenta 

 comprise what are termed dust, ashes, sand, lapilli, pumice, and 

 scoria, with fragments of stone ; but the latter category includes 

 them all, the difference being only that of size. The ashes therefore 

 consist of small fragments of lava comprising minerals of the 

 nature of felspar, augite, olivine, biotite, magnetite &c. Many of 

 these are opaque or coloured, and traces of their crystalline form 

 are very frequently visible. It is evident that these minerals 

 must be abundant or scarce, or one prevailing over another 

 according to the nature of the rock from which they are derived ; 

 but it is astonishing how one peculiar kind of mineral will prevail 

 over a wide area. 



Generally speaking ashes may be classified according to the rock 

 formation of the volcano. Most readers are aware of the great 

 divisions that are made between the acid or basic lavas as they 

 are called. These fall into five great groups of rocks viz. : the 

 rhyolitesor acid lavas, the basalts or basic lavas and the intermediate 

 lavas known as trachytes, andesites, and phonolites. The basic 

 lavas contain a larger proportion of oxide of iron and other heavy 



