BY H. I. JENSEN. 99 



There are probably two series of andesites, the first erupted 

 simultaneously with the intrusions of Point Arkwright porphyrite 

 and Mt. Cooroy monzonite, and closely allied in mineral composi- 

 tion to the monzonite; the second took place after the rhyolites 

 and trachytes, and before the basalts, and varies in constitution 

 from a dacite to a very basic andesite. 



The green andesitic breccias found at Yandina, and at Mt. 

 Eerwah, probably belong to the second series, for no trachy- 

 rhyolite dykes have been observed in them though fragments of 

 a trachytic nature abound. 



(4) Basalts . — The top of the Nindherry Ridge north of the 

 saddle is basaltic. Basalts cap the Woondum Tableland, Pin- 

 barren Mountain, and many points in the Kin-Kin and Wahpunga 

 Ranges. Here they overlie rocks older than Trias- Jura. 



In the Blackall Range we have an extensive area of basaltic 

 rock capping sandstones, rhyolites and trachytes to the thickness 

 of many hundred feet. Dykes of basalt penetrate the sandstone 

 on the eastern slopes of the range. The lowest layer of the 

 Blackall basalt is everywhere highly vesicular and amygdaloidal, 

 and often associated with basic tuff. The more rapid weathering 

 of this layer leads to the undermining of the upper portions, and 

 the formation of steep precipices of columnar basalt round the 

 Blackall tableland. The basaltic lavas here cover an eroded 

 surface, infilling original valleys (Text figs. 18 and 19). 



Buderim Mountain and Sippy Mountain (a hill S.S. W. of 

 Buderim), are also basaltic, and basalt dykes are frequent in all 

 the district between the Blackall and Mooloolah Heads. 



Mt. Mee has a basaltic capping covering an eroded surface of 

 hornblende schist, mica-schist, and, at Sellin's Creek, trachy- 

 rhyolite 



Age and Succession of the Lavas — (1) Simul- 

 taneously with the injection of sills and laccolites of quartz-diorite, 

 porphyrite and monzonite at Noosa, Pt. Arkwright, Cooroy, &c, 

 the first series of andesites and andesitic tuffs were erupted. At 

 Rumundi Railway Station may be seen a conglomerate of rounded 

 porphyrite and andesite pebbles in a sandstone base of apparently 



