BY H. C. RICHARDS. 187 



Lower Volcanic Rocks. 



These consist of basaltic agglomerate and basaltic 

 flows Avhich extend from a height of about 1 ,050 feet above 

 sea level to a maximum height of 1,400 feet on Mount 

 Boorambool, and a minimum height of 1,160 feet on Gill's 

 Hill, before they are succeeded by the trachyte tuffs and 

 trachytic flows which, in their turn, have been succeeded by 

 a series of basaltic flows. This basal agglomeratic material 

 is very AAell developed in the southern portion of the town 

 where it is shown in the streamcourse under the road bridge. 

 The agglomerate is made up of fragments of scoria which 

 range in size from that of a walnut to that of a large apple. 

 The fragmental material is bright in appearance and shows 

 a marked contrast in colour to the grey-blue lava with which 

 it is associated. Chalcedony is very abundant all through 

 the weathered flows and scoria, p,nd the junction line between 

 the lava and agglomerate is the most favoui^^ble spot for 

 its development. 



Evidently from the presence of agglomerate here, there 

 must have been a centre of explosive activity in the 

 immediate vicinity. Similar agglomerate is met A\ith on 

 the southern slope of the Mountain which is a huge trachytic 

 mass situated north-east of the town. 



Scattered about on the weathered surface of the lower 

 basaltic rocks are abundant pieces of chalcedony which. at 

 one time occupied the vesicles, etc. in these rocks. Many 

 of these fragments show opaline characters a.nd occasionally 

 closely approach precious opal. 



The flows exhibit a marked platy character and this 

 is due mainly to movement of flow during the consolidation 

 period as microscopic investigation shows the augite well 

 ■drawn out and with the lath-shaped plagioclase crj^stals 

 .all through it. 



Specimen 278*, from a well sunk in the yard of Scott's 

 Hotel, Springsure, is a dense fine-grained, greenish-black 

 plagioclase basalt. It has a density of 2.822 which is high 

 and, no doubt, largely due to the great amount of augite 

 and iron ores which the rock contains. Occasional amygdules 

 of chalcedony occur. 



*This number and others subsequent refer to the numbers in the 

 collection in the University of Queensland. 



