192 THE GEOLOGY OF THE CAXOBOLAS MOUNTAINS^ 



In many of the rocks the evidences of the solvent and mineral- 

 ising powers of circulating acid vapours in the period of consolida- 

 tion are well marked. The miarolitic nature of the plugs and 

 dykes is also indicative that the magma was very hydrous. 



In many of the comendites, quartz is not visible, but it exists 

 nevertheless in the base in the form of opal or chalcedony, and 

 tridymite. 



The sequence is the same as in the Warrumbungles. The 

 basic andesites cover the highest summits, and the basalts were 

 erupted last, apparently from fissures intersecting the country 

 surrounding the central area. 



Many rare minerals have been identified in small amounts 

 with considerable certainty. They comprise melilite, fayalite, 

 astrophyllite, perofskite, wohlerite, laavenite, kataphorite, and 

 cossyriteC?). 



Some of the rocks described take a beautiful polish and would. 

 make excellent and most durable building-stones. 



No basic tufi*s, lamprophyres or basic dyke- and sill-rocks have 

 been met with. The clastic and hypabyssal rocks all belong to* 

 the leucocratic and melanocratic trachyte series. 



General Note. — The volcanic rocks rest in this area on a pene- 

 plain, having an altitude of about 3000 feet The basalts were 

 erupted in some cases after the uplift and cailon-formation of the 

 present cycle of erosion. 



It has been shown by one of us (H. I. Jensen) that the Nan- 

 dewar lava-flows to the west of Mt. Lindesay rest on the sand- 

 stone at an altitude of 1,800 to 2,000 feet. To the east of Mt. 

 Lindesay and of a line running through it in a JST.N. W.-S.S.E. 

 direction, the lavas commonly have an altitude of 3,000 feet. 

 Between the mountain-region and Narrabri the mesas average 

 about 1,400 feet in altitude. It was shown that these discre- 

 pancies in the altitude of the peneplain are probably caused by 

 a fold (monoclinal) due to the uplift of the New England seg- 

 ment, and subsidence of the earth-segment to the west, and 

 further uplifts due to intrusions of igneous rock. This was 

 followed by the extrusion of lavas, and subsequently step-faulting 

 [Printed olf July 9th. 1909,] 



