BY H. I. JENSEN. 577 



does, however, overlie trachytes and ceratophyres in many places,* 

 and to the north of the main basaltic area we again come upon a 

 group of alkaline lava-masses extending from Nambour to 

 Cooroy, and from the Mary River to the sea. As I have shown 

 in my paper on this area (loc. cit.) the alkaline series is more 

 complete here than in the Glass House Mountains proper. 

 Volcanic activity commenced with the intrusion of huge sills and 

 laccolites of porphyrite and quartz-diorite (andose) and monzonite 

 (dacase) at Point Arkwright, Noosa Head, Mount Cooroy, and 

 elsewhere, and the simultaneous eruption of andesitic and dacitic 

 lavas in the Eumundi district. All these eruptions were of an 

 alkaline nature, as my analyses show. From the larger masses 

 were differentiated dykes of a beautiful graphic granophyre. and 

 a curious characteristic of all the hypabyssal and volcanic rocks 

 of this series is the almost universal occurrence of intergrowths 

 of minerals, graphic, micrographic, and cryptographic. Even 

 allotriomorphic phenocrysts having roughly the contour of felspar 

 and consisting of a micrographic intergrowth of quartz and felspar 

 occur. This shows that, cooling proceeded in the magmatic 

 reservoir until a number of phenocrysts had separated, and only 

 a eutectic mixture of quartz and felspar was left. Then the 

 magmas were expelled and cooling continued where the masses 

 occur at present. Felspar being greatly in excess of quartz in 

 the mixture, and the original conditions having been disturbed, 

 the felspar molecule seems to have reacquired a tendency to 

 crystallise, hence we get the remarkable appearance of micro- 

 graphic phenocrysts in a micrographic to granular base. 



The next lavas to be expelled after a period of erosion were 

 acid and very alkaline rhyolites and trachytes, with arfvedsonite, 

 riebeckite, and cossyrite. Then followed alkaline andesites with 

 segirine-augite and acmite; next dacites; last alkaline to normal 

 basalts. 



Now the whole volcanic series from Mt. Byron to Cooran lies 

 on a long earth-fracture preserving a general north and south 



* ** Geology of the East Moreton and Wide Bay Districts." Proc. Linn. 

 Soc. N.S. Wales, 1906, Part 1. 



