576 THS DISTRIBUTION, ETC., OF ALKALINE ROCKS, 



that it is unnecessary, as differentiation alone is capable of 

 explaining all facts. I do not think so. A powerful argument 

 against the assumption that foyaitic and calcic rocks are derived 

 from one parent-magma and have then been further differentiated 

 into acid and basic varieties of each is found in the inf requency of 

 passage-types and the distinctive dyke-series which accompanies 

 each of these two main magmas. Another is the occurrence in 

 alkaline rocks of so many minerals which are characteristic of 

 metamorphic rocks. 



Queensland. — Proceeding to Queensland, we first meet with 

 allied alkaline rocks at Mt. Flinders near Ipswich; these are 

 fully described in a subsequent chapter. 



At Spring Bluff, between Helidon and Toowoomba, trachytes 

 underlie basalts, but not having examined sections of them I 

 cannot say if they are very alkaline or not. They support a 

 flora similar to that occurring on the ceratophyres of Byron 

 Creek, grass-tree playing an important role. 



On the North Coast Line, between Brisbane and Gympie, we 

 come upon the alkaline areas of the Glass House Mountains. In 

 the Glass House proper the alkaline series is not very complete. 

 The leucocratic rocks are the most abundant and comprise 

 comendites, pantellarites, arfvedsonite and aegirine trachytes, 

 and occasional ceratophyre dykes. 



Some of the more southerly mountains are composed of a por- 

 phyritic bostonite. The melanocratic alkaline rocks include 

 dark, very fine-grained segirine trachytes and dacites. The 

 subsequent basalts are calcic. 



The order of eruption was : — 1. Acid alkaline lavas. 2. Dacitic 

 lavas. 3. Basalts. 



The basalts, however, were not erupted iu the same district, 

 but to the north of the main trachyte-area The original vents 

 seem to have been effectually plugged up by the pasty trachytes, 

 and the basalts were compelled to find an outlet by fissures in 

 the area to the north. The great Blackall Range basalt-sheet 



