132 VOLCANIC ROCKS OP SOUTH-EASTERN QUEENSLAND 



somewhat definite lines. In the Springbrook and Tam- 

 borine areas however, fissure eruptions were most probably 

 responsible for the extensive development of lithoidal 

 rhyolite which is found there. All stages from holocrys- 

 talline to thoroughly glassy rocks are represented, and 

 there is a great diversity of types. In the lava flows both 

 the porphyritic and lithoidal varieties occur, though the 

 latter are the more abundant. 



Definite flows are found building up portions of Tam- 

 borine Plateau, Springbrook Plateau and MacPherson's 

 Range from Mount Cougal on the east nearly to Mount 

 Wilson on the west. Along the Coomera River there are 

 also extensive lava flows. A general idea of the thickness 

 of these flows has already been given. 



Large masses which seem to represent plugs associated 

 with central vents occur at Miount Lindsay, Mount Barney, 

 Mount Maroon, the Maroon Range, Knapp's Peak, Mount 

 Moon, Mount Alford, in the Esk district, particularly at 

 Glen Rock, and in the Glass House Mountains district. 

 Dykes are not frequently met with, but in the Cunning- 

 ham's Gap area there are several. In Johnston's Creek 

 below Mount Matheson there is a rhyolitic dyke striking in 

 a north-west and south-east direction ; its width cannot be 

 determined as it passes under alluvium on the east side, 

 but it is at least 50 feet wide and its western margin is a 

 dark pitchstone. This pitchstone margin is several feet in 

 thickness, and is somewhat porphyritic. Near Mount 

 Alford, both pitchstone and rhyolitic dykes occur, and 

 just near the Moogerah School an excellent pitchstone dyke 

 is seen. 



On the Upper Logan River between Mounts Lindsay 

 and Barney, there are rhyolitic dykes intruding the "Walloon 

 sandstones and shales, and at the Yellow Pinch reserve, 

 just to the east of Mount Barney, there is a rhyolitic dyke 

 about 15 feet wide, which has severely baked the intruded 

 sandstones and shales. 



Heat metamorphism of the intruded coal-measures 

 seems to have been more pronounced in connection with 

 the Mount Barney mass than in any other locality examined. 



