BY H. I. JENSEN. 861 



2. The TiLassic or Trias-Jura rocks, consisting of sandstones 

 and conglomerates, with interbedded shales and mudstones. 



The igneous rocks may be divided into Plutonic and A^olcanic, 

 the dyke rocks being best considered with one or other of these 

 divisions. 



(a) Plutonic. — To this division belong the granites, diorites, 

 gabbros, gneisses and augen-gneisses of the coastal range. These 

 ancient rocks form the core of the range, being flanked on the 

 eastern side by the Palaeozoic slates and schistose rocks. They 

 are cut by dykes of aplite, segerine S3'enite, hornblende rock, 

 diabase, &c., as well as by quartz reefs and leaders, usually more 

 or less metalliferous. 



(b) Volcanic. — The volcanic rocks of the district fall petro- 

 logically under three heads — trachytes, andesites, and basalts. 



(L) Trachytes. — The Glass House Mountains proper are all 

 composed of trachyte. To make clear the mineral constitution 

 of the most typical rocks, it will be useful to consider first a few 

 special cases. 



Bperhurrnm Trachyte. — Hand specimens of this rock when 

 freshly broken have a glistening white marbje-like appearance. 

 On decomposing the rock acquires a reddish, or dull brick-coloured 

 tint. The glistening of fresh specimens is due to large crystals 

 of sanidine, up to J an inch in diameter. The rock is seen to be 

 coarsely porphyritic, and to consist almost entirely of felspar,. 

 ferromagnesian minerals being only revealed on examination with 

 a pocket lens. 



Microscopic examination showed that the base consisted of 

 lath-shaped, felspar crystals, of the sanidine variety, and two 

 varieties of hornblende; the one hornblende is deep brown to 

 reddish-brown in colour, strongly pleochroic, and more plentiful 

 than the other variety, which is a deep blue pleochroic hornblende, 

 probably riebeckite. A colourless pyroxene, non-pleochroic and 

 with strong birefringence, is also present in scattered irregular 

 grains. 



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