BY H. I. JENSEN. 857 



Very little denudation has taken place since its period of 

 activity, although the rock is very decomposable. Its distance 

 fromi the nearest basaltic mountains of the Blackall Ranges is 

 about five miles, and if it represents a remnant of a denuded flow 

 from them, a mass of l^asalt over 800 feet in thickness has been 

 removed in the valley between them. If that were the case, it 

 is hardly imaginable that we should meet with such excellently 

 preserved vesicular basalt on the very summit of Mt. Mellum.* 



The balance of evidence, therefore, favours the supposition that 

 it represents a volcano. 



(9) Situation of the Volcanic Mountains on Intersecting Growps of 

 Cracks. — By looking at the accompanying map (Plate xlvi.) it will 

 be seen that the mountains of the Glass House group lie on inter- 

 secting cracks, having approximately the directions N. to S. and 

 E. to W. The main fissure seems to be that on which Miketeebu- 

 mulgrai, Tunbubudla, Conowrin and Mt. Mellum lie. Another 

 line may be drawn in a nearly parallel direction through Beer- 

 burrum, Tibrogargan, Ngun Ngun, Coochin Hill and Mt. Mellum. 

 At right angles to these two lines we find one passing through 

 Beerwah, Conowrin and Ngun Ngun ; a parallel fissure passes 

 through Mt. Beerburrum and the two Tunbubudla mountains. 



The dykes radiating from Tunbubudla may be looked upon as 

 radial cracks caused by the lava outburst. 



(10) Age and Origin of the Glass House Mou7itains and 

 adjacent Rocks. — There is no evidence that the Glass House 

 Mountains have been submarine in origin. There are no sub- 

 marine tuffs; the holocrystalline nature of the trachytes, as well 

 as the occurrence of large fragments without any definite orienta- 

 tion in the Trachyte Range tuffs, and the absence of definite 

 arrangement of the crystals in these tuffs are evidence against 

 submarine origin. In the trachytic lavas, too, we meet with but 



* " Mellum " seems to be an aboriginal word for volcano. The mountain 

 may have been active in the human period. Otherwise, why should the 

 natives have given it the present name ? Rumblings are said to have been 

 heard under it last year. 



