BY H. I. JENSEN. 855 



covering of vesicular volcanic rocks, Bullawa Creek itself flows 

 from its junction with Oakey Creek through sandstone and 

 trachyte sills. Sandstone, shale, dolerite, trachyte, and many 

 other rocks chiefly related to the trachyte family occur in the 

 broken country between it and the tableland. 



At Dingo Creek most interesting features were observed. 

 Not far (about 250 yards) from its junction with Bullawa Creek 

 there is a remarkable sill of lamprophyric porphyry intruded 

 along a coal seam (S^). The dip of the strata is S.E. at 25°. 

 Below and above the sill there are Glossopteris and Noeggera- 

 thiopsis shales and cherts indurated by the intrusion. 



About 500 yards further up the creek we meet with the 

 boundar}' of a great laccolite of basic porphyrite intruded by 

 occasional trachyte dykes. As I have seen sandstone both over- 

 lying and underlying this rock, I have no doubt it is a laccolite. 

 Sandstone dips are somewhat disturbed near its edges. Further 

 up the creek the basic rock is seen sometimes only on the east 

 bank, sometimes on both banks, and the creek has largely 

 carved its way along the western edge of the mass. On the 

 west bank of the creek, cliffs of sandstone with coal seams and 

 Glossopteris shales are common. They dip S.E. at 25-30° and 

 contain interlaminated sills of trachyte porphyry and bostonite, 

 which generally have penetrated along a coal seam. The hills 

 west of Dingo Creek are capped with trachyte, but those on the 

 east side for some distance consist chiefly of dolerite, with 

 trachyte cappings overlying it in places, a fact which proves 

 that denudation had removed the sandstone covering of the basic 

 laccolite before the trachytic rocks were poured out. The dolerite 

 is of various degrees of crystallinity, some very coarse-grained, 

 some, especially near the edges of the mass, fine-grained. 



The ridge between Dingo Creek and the creek west of it owes 

 its existence to a broad trachyte dyke running S.E.-N.W., from 

 which many sills and dykes at right angles to it and flows 

 capping the hills are derived. The basic laccolite has a maximum 

 thickness of about 700 feet (Fig. 4). 

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