184 VOLCANIC ROCKS OF SPRINGSURE. 



accumulations of volcanic rocks. The Opal Ridge aiid 

 Gill's Hill consist of trachyte and trachytic tuff rising to a. 

 height of 1,260 feet above sea level on Gill's Hill, i.e., approxi- 

 mately 200 feet above the railway station level. 



Mt. Boorambool further west from the town than Gill's 

 Hill is about 2,000 feet above sea level and has a level top 

 several hundred yards long and in width from 20 to 100 

 yards. The length of the ridge is along a north and south 

 line. Mt. Boorambool is very steep and this is due to the 

 hard protecting cap of olivine basalt which weathers less 

 readily than the underhing volcanic rocks {see Text-Fig 27). 



Photo. H. C. R 



Text-Fig. 27. — Mt. Boorambool from the Tambo road near the OpaJ 

 Patch. The level surface of the ba.salt capping is well seen. 



Further to the west a ridge on a more extensive scal& 

 but with a similar mesa-like character occurs. Several 

 isolated mesas with basalt cappings occur to the south and 

 south-west. The question ari.ses as to whether these points 

 were all connected at one time and subsequent denudation, 

 has left them as residuals or whether they are the residuals 

 of several flows in different valleys. The latter view is 

 the more probable. 



The Mountain which rises to 2,000 feet a])ove sea level 

 is a mass of trachyte resting on basaltic agglomerate and 

 basalt. Its sides are very precipitous and it is a matter of 

 considerable difficulty to ascend the mass. The height of 

 the Mountain al)ove the surrounding country is about 

 1,000 feet and the upper 600 feet are of trachyte. The top 



