ON THE GEOLOGY OF WEST MORETON, QUEENSLAND. 143 



Mount Flinders. — The trachyte series at Mount Flinders 

 (2240 feet) can be subdivided into three distinct sub-periods 

 of eruption. The first produced the dark basic looking 

 trachyte (pantellarite of Jensen). It has a characteristic 

 greasy looking lustre much like phonolite. The lower hills 

 on the the northern side of Flinders are composed of this 

 rock. The second sub-period produced the light alkaline 

 felspar porphyry which composes Flinders and a number of 

 the neighbouring peaks. Two distinct dykes of this light 

 trachyte run from Flinders through the pautellarites, one 

 to the north-west of that mountain, 20 feet wide, shewing 

 well defined horizontal prismatic structure. The third 

 sub-period produced a pitchstone porphyry containing 

 phenocrysts of sanidine embedded in a black glassy matrix. 

 (See petrographical descriptions.) 



Mount Blaine about two miles to the north of Mount 

 Flinders is composed entirely of this material with inclu- 

 sions of light and dark trachyte. One inclusion of the 

 light variety measured 6" x 4", and another of pantellerite 

 5" x 4". 



Ivory's Rock which can be seen about three miles to the 

 east of the Rockton Railway Station, standing like a large 

 obelisk above the plain, is about 1,300 feet high, and is 

 composed entirely of trachyte breccia. At a point 400 

 feet from its summit the angular masses of breccia are 

 cemented in a matrix of trachyte glass which seems to 

 have forced its way from the centre of eruption through 

 the porous masses of scoria. 



Main Dividing Range. — The main range near Cunning- 

 ham's Gap is composed of alkaline trachyte capped by 

 olivine basalt. Mount Matheson (2,660 feet) appears to 

 have been the main focus of the trachyte eruption of this 

 district. Its summit consists of vesicular trachyte sur- 

 rounded on all sides by huge masses of trachyte tuff, breccia 



