48 
GEOLOGY OF THE GLASSHOUSE MOUNTAINS. 
may presume that this isolated group of hills represents the site 
of ancient volcanoes which burst through fissures in the sand- 
stone and shales of the Jurassic period, and the solid plugs of 
trachyte which filled the throats of the original cones alone 
remain as evidences of their former greatness. The loose 
accumulations of ejected materials that we may assume at 
one time existed on their flanks have been stripped oft' and 
swept away during ages of denudation, only the more compact 
fragmental material being left at their base. Thus the ridges 
in the vicinity of Coonowrin and Tibberogargan, two of the 
principal elevations composing the Glasshouse ]\Iountains, 
consist mainly of solid trachj^te and trachyte-breccia, with 
fragments of coarse sandstone and grit strewn about the 
surface. The rock generally is a light coloured, rough and 
porous, but fine-grained variety of trachyte, rich in sanidin, 
with numerous needles and prisms of hornblende, plagioclase, 
some biotite and porphyritic blebs of quartz. 
The sides of Beerwah and Coonowrin present very beautiful 
examples of columnar structure. Both vertical and inclined 
columns are present, and these are mostly hexagonal and do 
not a]Dpear to exceed 2 feet in diameter. As the rock also 
weathers unevenly in places, shallow cavernous hollows are 
thus formed in the more exposed sides of the mountains. 
Wherever conditions have been favourable for the accumulation 
of rock debris and soil, vegetation is abundant; but there are 
many quite naked rock exposures and escarpments. From these 
remarks it may be affirmed that this recent examination supports 
the conclusion respecting the nature and origin of these remark 
Jle landmarks in southern Queensland previously arrived at by 
the ill-fated Ludwig Leichhardt. 
