BY H. C. RICHARDS. 159 



of Spicer's Gap, on the eastern side of the range; at Mount 

 Alford forming the base of the mountain; at Tamborine 

 Plateau wliere there is an extensive and well-defined flow 

 near the top of the plateau, and seen particularly well at 

 the south-west end ; at the summit of Mount Meerschaum ; 

 at the Lookout on ^MacPherson 's Range where the Spring- 

 brook Plateau terminates ; and at One Tree Hill near Point 

 Danger. Just over the State border fence from One Tree 

 Hill, similar material forms Observation Hill, and this, 

 although in New South Wales, will be dealt with here. 



It is found advisable to treat these rocks in tW'O divi- 

 sions, one embracing the augite-andesites from the Esk 

 district, and the other the more normal sub-alkaline 

 andesites. 



Angite-Andesites. 



These occur near the Ottaba railway station, and also 

 along the railway line towards Esk. Sections are seen 

 Aviiere they intrude through the Walloon shales. At a 

 cutting 42 miles 62 chains distant from Ipswich there is 

 an excellent section showing an intrusion of andesite which 

 has had a severe baking effect on the intruded shales. 

 There are several intrusions seen along the line, and in 

 the cutting immediately to the south of Ottaba there is a 

 very large one, several hundreds of feet in width. The 

 freshest material found was in poi'tion 28v, parish of 

 Biarra, and its description will serve for aU this material. 

 An analysis has been made of the andesite from portion 

 134, parish of Esk, and the sample was collected from the 

 railway cutting. In the hand-specimen the rock is a light 

 grey with abundant phenocrysts of augite which have a 

 length of 2 or 3 mm. ; around these there are accumulations 

 of small granules of augite and this gives the rock a rather 

 mottled appearance. In addition to the augite phenocrysts, 

 there are plagioclase phenocrysts up to 2 mm. long. Micro- 

 scopically the groundmass is seen to be hyalopilitic and 

 to contain plagioclase, augite and magnetite. Here and 

 there are rounded patches up to -75 mm. in diameter and 

 these are made up of small granules of augite. The augite 

 phenocrysts frequently show twinning and are usually 



