BY H. I. JENSEN. 79 



west of it. Yet that old sea-area stands now 2,000 feet higher 

 than the old land-area. How can we explain this phenomenon 1 

 Evidently at the end of the Cretaceous period the Cretaceous sea 

 dried up, and its basin was uplifted, the expansion of Triassic 

 and Jurassic sediments aiding to bring about vertical uplift. 



Possibly, as the sketch (figs, 2-3) suggests, the Ipswich and 

 Walloon Measures participated in the uplift and folding, which, 

 however, were greatest over the Downs basin, where sedimenta- 

 tion was thickest. Why now have we the Walloon coal-measures, 

 which, on the Darling Downs, immediately underlie the Creta- 

 ceous, lying about 2,000 feet lower in the Ipswich area than in 

 the Downs, and dipping west in under the Darling Downs 1 

 Either the Walloon measures are very thick, exceeding 2,000 feet, 

 and a considerable thickness has been removed by erosion east of 

 the range, or a Post-Cretaceous fault has lowered the whole area 

 over which they now outcrop by about 2,000 feet. 



The first hypothesis I object to on the follow^ing grounds : — 



(1) I believe that 2,000 feet is an overestimate of the thickness 

 of these beds. 



(2) If this area was land in the Cretaceous, as it appears to 

 have been, the slight angle of dip of the beds of the Walloon 

 series, and the continuation of apparently the same beds higher 

 up in the Darling Downs tableland, and the existence of what 

 are looked upon as Cretaceous deposits in patches formed by 

 aggradation, are facts implying that the area formed in Cretaceous 

 times a lowlying coastal plain fringing Cretaceous sea, and that 

 it consequently did not suffer extensive Cretaceous erosion. But 

 in early Tertiary times a fault developed and the country west of 

 it was progressively raised, and that east of it depressed. Simul- 

 taneously volcanic outpourings overspread the plains on both 

 sides of the fault. Thus we have trachytes, trachyte- tuffs^ 

 basalts, and andesites capping the Walloon coal-measures in many 

 places. These lavas probably commenced to be erupted in Eocene 

 times, and have protected the underlying Trias-Jura from Tertiary 

 erosion. If Tertiary erosion had been very pronounced, it is 

 doubtful whether any Eocene tuffs would have been preserved to 



