252 PROCEEDINGS OF SECTION C. 



The members of tke Maroochy-Cooran Group are scattered about 

 in an indiscriminate way, and it is impossible to detect any definite 

 arrangement. Tlie countiy is considerably faulted, but o^ving to its 

 level nature, the denseness of the vegetation, and the absence of mining 

 shafts, it is impossible at present to map the faults. 



The mountains of the Glass House Group lie on two sets of inter- 

 secting fractures. The main fracture runs S.S.E.-N.N.W. ; and numer- 

 ous fault lines cross this almost at right angles. Radiating dykes in 

 sets diverging from the principal foci of eruption form a noteworthy 

 feature in this district. 



The hills around Mount Flinders are probably situated on fissures 

 radiating from the main focus of activity. 



The Little Livei-pool Range has the same trend as the D'Aguilar 

 Range, viz. : — N.N.W.-S.S.E.," and in the arrangement of the trachytic 

 peaks of this area an indication of sets of intersecting cracks, as in the 

 Glass House Mountains, is noticeable. 



One of the most characteristic features of a mountain of alkaline 

 rock is its ruggedness and precipitousness. This is pai-ticularly so for 

 those composed of comendite, or other leucocratic types of alkaline 

 rock. Some alkaline masses form rugged steep cones ; such sugarloafs 

 are Mount Tunbulmdla, Coochin Hills, Micketeebumulgrai, &c., in the 

 Glass House Group; Mount Edwards, Mount Greville, &c., in the 

 Fassifeni Group. Trachyte and comendite lavas are of a veiy viscous 

 character, and would cool with very steep sides if slowly extruded from 

 a small vent. It appears, therefore, that most of the sugarloaf-shaped 

 hills and many of the steeper pinnacles have originated in the same 

 way as the mamelons of Mauritius. In other cases the steep obelisks 

 and pillars may have originated by the extrusion of a solid column of 

 congealed lava like the new lava pillar of Mount Pelee in Martinique, 

 but in most cases the veiy steep, precipitous, and almost inaccessible 

 trachyte peaks have originated by the intiaision of a lava mass into 

 the pipe of a tuff cone, and the subsequent removal of the tuffs by 

 denudation. This process of destruction of tuff cones, leading to the 

 survival of the volcanic neck only, I have observed in various stages 

 at Mount Flinders, Mount Coolum, Mount Beenvah, and elsewhere. 



Columnar structure is another almost invariable feature of the 

 leucocratic trachytes and comendites, and as already pointed out in 

 my previous papers, the arrangement of the columns is such that 

 there can be no doubt that the trachytic peaks are independent foci 

 of eruption. The arrangement shows, too, that the peaks are not 

 " monadnocks," and it favours the theory that they constitute the 

 necks and plugs of tuff cones. 



4. Petrography. — The rocks of the alkaline areas may be divided 

 as follows : — 



A. — Sedimentary. 

 B. — Metamoi-phic. 

 C. — Igneous but not alkaline. 

 D. — Igneous alkaline. 

 In the Maroochy-Cooran and tlie Glass House areas all these 

 rock classes are present ; but at Mount Flinders and in those parts of 

 the Little Liverpool and Fassifern areas which I have visited rocks 

 affected by regional metamoi"phism are absent. 



