alkaliis^e rocks of s. queensland. 257 



7. The Age op the Alkaline Eruptives and the Volcanic 

 Sequence. — No definite evidence as to age has been observed except 

 that the alkaline series has intruded the Upper Trias-Jura. As similar 

 rocks occur in New South Wales (the Warrumbungle Mountains group) 

 of Lower Tertiary, probably Eocene age, it is likely that the Queens- 

 land alkaline rocks are of the same age. 



The sequence is everywhere the same. The alkaline rocks were 

 first extruded. Andesites and dacites followed close after in some 

 localities; a period of erosion followed; after which basalts were 

 erupted. As the South Queensland basalts are considered Pliocene, 

 and a period intervened between the trachytic and basic eruptions, 

 this fact too tends to show that the eruptions took place in the early 

 Tertiaiy, Eocene, or Lower Miocene. 



8. Soils and Economic Notes. — The alkaline rocks everywhere 

 give but poor soils. Where basaltic eruptions have followed the 

 alkaline good countrj^ predominates. 



Many of the columnar trachytes would make beautiful and most 

 durable building stones. Particularly fine stone could be obtained 

 from Mount Cooran, Mount Ngun-Ngun, Mount Conowrin, and Mount 

 French. 



Alum caves exist near the summit of Mount Flinders. The alum 

 occurs as veins in breccia, and is probably formed by the decomposi- 

 tion of pyrites and felspar. 



Coal occurs in the Trias-Jura beds in all the districts noted for 

 alkaline rocks. It is possible that the heat of these slowly cooling 

 igneous rocks may have produced petroleum oil in the coal beds 

 invaded by them. Boring for oil in these districts might meet with 

 a successful termination. 



9. References to the author's published papers on the subject- — 



(a) " The Geology of the Glass House Mountains," H. I. Jensen. 

 Proc. Linn. Soc, N.S.W., 1903, Part 4. 



(6) "The Geology of the Volcanic Area of the East Moreton 

 and Wide Bay districts, Queensland," H. I. Jensen. Proc. 

 Linn. Soc. of N.S.W., 1906, Part-. 1. 



(c) " The Distribution, Origin, and Relationships of Alkaline 



Rocks," H. I. Jensen. Proc. Linn. Soc. of N.S.W., 1908, 

 Vol. XXXIIL, Part 3. 



(d) '■ The Alkaline Petrographical Province of Eastern Aus- 

 tralia." Proc. Linn. Soc, N.S.W., 1908, Vol. XXXIIL, 

 Part 3. 



(e) " The Geology of Mount Flinders and the Fassifem." 



if) " Note on a Glaucophane Schist from the Conondale Range, 

 Queensland." Proc. Linn. Soc, N.S.W., 1907, Vol. 

 XXXII., Part 4. 



