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



(2) Constituents (in order of decreasing abundance). — (a) Lath- 

 shaped felspar, including (i.) orthoclase and (ii.) olioclase with 

 albite twinning and extinction angles up to 5° or 6°; (b) idio- 

 morphic cubes of magnetite, (c) lath-shaped crystals of segirine 

 decomposing to chlorite, (d) hematite from the decomposition 

 of segirine; (e) quartz (a little), (f ) apatite, (g) kaolin from the 

 decay of the felspar. 



Order of crystallisation normal. 



Name. — This rock occurs in a fairly large mass, which is 

 probably not of true volcanic origin; yet it has cooled so near the 

 surface that it has as many affinities with the volcanic as with the 

 hypabyssal rocks. Quartz- Andesite seems to be the most suitable 

 name. The segirine shows that it is rather alkaline. 



A closely allied rock is Sp. No. 18, which occurs as a large 

 intrusive mass in the cutting at the 66^ m. peg, on the railway 

 line between Yandina and Nambour. 



Sp.No.152. Sodic Quartz- Andesite or Dacite. 



Obtained on the railway line between Eumundi and Cooroy, is 

 similar macroscopically. Under the microscope we find it the 

 same in texture, but the constituent minerals are as follows : — 

 (a) felspar, comprising andesine and oligoclase; (b) magnetite, 

 (c) segirine, (d) chlorite, (e) augite surrounded by a rim of hematite, 

 {/) a little hornblende decomposing to chlorite, (g) a little quartz, 

 orthoclase and apatite. 



Evidently it is a dacitic rock, closely allied to Sp. No. 163. 



Sp. No.165. A somewhat decomposed andesitic rock with green 

 specks, occurring as a dyke on the Kenilworth Road, ^ m. from 

 Eumundi. In texture it is like the preceding. In constitution, 

 however, it has been a much more typical andesite. It consists of 

 plagioclase; magnetite and chlorite, both secondary after horn- 

 blende; and original magnetite. 



Sp..No.l67. Andesite. Loc: Hull's Selection, Mt. Cooroy. 



i. In handspecimen a dark, compact, greenish rock with felspar 

 phenocrysts. 



ii. Under the microscope, seen to be a hypocrystalline, uneven, 

 but fine-grained rock, with hyalopilitic fabric. The chief con- 



