96 VOLCANIC AREA OF EAST MORETON, ETC., DISTRICTS, Q., 



Mountains, is not certain. The latter hypothesis is the more 

 probable. 



The Rhyolites and Orthoclase Porphyries of 

 Neurum and Delaney's Creek. — Mt. Archer, Mt. 

 Delaney, Black's Hill and many smaller eminences in this area 

 are composed of a very decomposed orthoclase porphyry, and a 

 rhyolitic rock which exhibits all degrees of texture from obsidian 

 to quartz-porphyry. These rocks are frequently interbedded. 

 Their volcanic origin is proved by (a) their association with tuffs; 

 (b) frequent banded arrangement and flow-structure; (c) amyg- 

 daloidal and vesicular structure in places; (d) by a breccia sheet 

 consisting of fragments of mica-schist, and granulite cemented 

 with rhyolite, underlying the rhyolite and porphyry capping of 

 Mt. Delaney; (e) by the occurrence of pitchstones and obsidian 

 in the masses. These rocks have differentiated from the same 

 magma, and have probably exuded from the same vent. They 

 have cooled rapidly. Many of the spherulitic types owe this 

 structure to devitrification. The columnar types are very like 

 those of the Maroochy district at Nindherry, Eerwah, and 

 Tin beer wah. 



These rocks overlie granites and Palaeozoic schists and granulites. 

 Many hundred feet of rock have been removed by denudation 

 since their ejection. The breccia at the base of the Mt. Delaney 

 flow is probably an old river-gravel. The eruptions took place 

 when the country was very rugged ; lavas filled the valleys. 

 Base-levelling subsequently formed the Woodford peneplain, and 

 the old valleys, protected by their lava-cappings, resisted erosion- 



The period of activity was probably the same as in the Glass 

 House Mountain and Maroochy districts. 



(2) The Trachyte Serie s. — Some new occurrences are 

 mapped on the Plan (Plate v.) at Barren Mountain, Bridge's 

 Hill (i), Bell's Quarry (ii), and west of Tunbubudla (iii) on 

 Battersby's. They are situated on linear fissures or cross-cracks 

 through other important members of the Glass House group. 

 Some new types of very amygdaloidal and vesicular trachyte 

 were met with at Med way's Mountain. The outer members of 



