330 THE DESERT SANDSTONE. 



eruption. The first or oldest, extends nearly all round the 

 continent, and at least 300 miles into the interior. The south- 

 west part of the Australian land seems to have escaped this 

 volcanic disturbance, which however extended to Tasmania. There 

 is a second period commencing towards the Pliocene epoch, 

 continued into the most recent post-Pliocene times. These 

 conclusions are entirely in accordance with those arrived at by 

 Mr. Selwyn and his officers in the Victorian geological survey. 

 They are also fully confirmed by Messrs. Daintree and Jack's 

 observations in Queensland. 



Mr. Selwyn's observations are thus stated *: — " The mineral 

 and lithological characters of the volcanic products of the two 

 periods present marked similarities and differences. An irregular 

 concentric concretionary, or polygonal jointed structure, is 

 eminently characteristic of the older, so much so that, though it 

 affords any quantity of excellent road-making or railway-ballasting 

 material, no building-stone is ever procured from it. The columnar 

 structure is also not uncommon in both the older and the newer 

 formations. Interbedded with the harder layers are strata of a 

 very soft, unctuous, amygdaloidal clay or ' wacke,' bluish-grey, 

 brown, yellow, brick-red, or pure white, Sometimes the section 

 exposed consists almost wholly of such clay, traversed by ferruginous 

 veins, and enclosing hard lumps or balls of dense-black basalt." 

 The solid layers are mostly a dense, dark, crystalline basalt, 

 composed chiefly of augite, labradorite, olivine and specular iron. 

 Though some are cellular they are far more solid and dense than 

 the recent Pliocene lavas, which present a much greater variety 

 of texture, the most common and characteristic being the well- 

 known Melbourne bluestone. It is a true dolerite or variable 

 mixture of augite, labradorite, iron and carbonate of lime, vesicular 

 and coarsely crystalline and compact, and not unlike a hard 

 calcareous or metamorphic sandstone of blue, grey, or almost black 

 colour. Olivine, specular iron, and hyalite are associated but not 

 as component parts of the stone. In some districts it is associated 

 with beds of regularly stratified ashes. These are so soft when 

 first quarried as to be easily sawn into blocks which harden 

 considerably after exposure. Obsidian is found, but no true 

 trachyte. 



The analysis of the older basalts shows a considerable proportion 

 of iron and magnesia, the iron being as high as 31 per cent, in 

 some lavas, and the magnesia 18. In the newer volcanic basalts 

 a similar character is prevalent in some of the dolerites. 



Mr. Daintree says in the essay on the geology of Queensland, 

 from which we have already quoted, that '' volcanic action seems 



* Notes on the Physical Geography, Geology, and Mineralogy of 

 Victoria, p. 29, by A. Selwyn, &c, Melbourne, 1866. 



