BY H. I. JENSEN. 855 



have altered the tuffs here and rendered them capable of resist- 

 ing denudation. 



(6) Occiirrence and Structure of the Andesite Formation. — 

 This formation covers an area of about 500 acres lying S.E of 

 Mt. Conowrin. The andesite assumes an irregular columnar 

 structure in places, and has to some extent prismatised the 

 underlying conglomerates. It is extremely variable in basicity, 

 being in some places pale grey in colour, in others perfectly black; 

 sometimes rather fine-textured, sometimes coarsely porphyritic. 

 The amount of quartz varies considerably. Perhaps the name 

 dacite will be found more appropriate than andesite. It is 

 important that it contains trachyte inclusions, hence is later 

 than the trachytes. The geburite-dacites of Mount Macedon, 

 Yic, were found to be the earliest trachytic rocks erupted in 

 that region, so the order of eruption is somewhat different in the 

 Glass House Mountains and Mount Macedon.* 



(7) Occurrence of Columnar Structure. — Mt. Conowrin displays 

 columnar structure on a grand scale. The summit is inacessible, 

 and consists of a mass of vertical trachyte columns. These are 

 square in transverse section (Plate xlvi., fig. 2). 



The Mountains Beerwah, Ngun Ngun, Tibrogargan, Cooee, 

 Ewin and Tunbubudla all show a central plug of columnar 

 trachyte similar to that of Conowrin. In the case of Ngun Ngun 

 we find, in addition to a mass of squarish columns exposed on 

 the S.E. side of the summit, that the main body of the mountain 

 is composed of huge columns of coarse-grained trachyte, rather 

 rich in iron-bearing constituents. These columns are polygonal 

 in transverse section ; they are vertical on the summit, but 

 horizontal or inclined on the sides of the mountain. A study of 

 the arrangement of the columns on Ngun Ngun somewhat 

 strengthens the idea that some of these mountains are of the 

 nature of mamelons. 



^' G-regory, J. W. (and Grayson, H. J.), "The Geology of Mount Macedon, 

 Victoria." Proc. Eoy. .Soc. Victoria. Vol. xiv. (New Series), p. 186, 1902. 



