180 VOLCANIC ROCKS OF SPRINGSURE. 



to carr}' out three complete rock analyses of the three main 

 types of volcanic rocks from the area, and a much more 

 accurate knoAvledge of the rocks is gained thereby. 



The similarity of the volcanic Pocks in their lithological 

 characters and in their arrangement with the volcanic rocks 

 of south-eastern Qiieensland is very marked, as there is a 

 three-fold dcA^elopment, with the alkaline trachytic rocks 

 forming the middle representatives while basalts constitute 

 the lower and upper divisions of the volcanic effusions. 



Dr. Jensen and others have referred to the area as one 

 ■containing alkaline trachytes and the area is regarded as 

 being near the northern termination of a fairly definite 

 line through Eastern Australia which connects the develop- 

 ments of Cainozoic alkaline trachytes*. 



Previous Literature. 



In 1804, in the Annual Progress Report of the Geological 

 Survey Dr. R. L. Jack gives a general account of the geology 

 of the Springsure district. Among other things he states 

 on page 9 : " The village of Springsure is very picturesquely 

 situated in the centre of a volcanic region biiilt up of 

 t^uccessive beds of lava, sometimes trachytic, but for the 

 most part consisting of basic rocks, varying from ordinary 

 basalt (sometimes glassy) to highly augitic andesite." . . 

 " Both the basic and acidic rocks contain, on joints and 

 faces and in cavities, immense quantities of hyalite and 

 chalcedony." Dr. Jack also refers to the opal-b^.aring 

 trachyte on a ridge just to the south of the town. 



Mr. C. F. V. Jackson, in j^ublication 177 of the 

 Geological Survey of Queensland, gives an excellent account 

 of the interesting development of opal in the trachyte^ 

 near Springsure. He writes : " The trachyte contains in 

 many places large numbers of vesicular cavities, small 

 thin veins and cracks, etc., filled with different varieties of 

 opal which have been formed by infiltration of siliceous 

 waters from the rock mass. The common forms are very 

 plentiful, but occasionally the opal in the cavities is of the 

 precious or noble variety, and gems so found are said to 

 exhibit very brilliant colouring." 



*Jensen, H.I., Proc. Linn. Soc, N.S.W., xxxiii, 1908, p. 585, text-fig. 

 10. 



