874 GEOLOGY OF GLASS HOUSE MTS. AND DISTRICT, 



sliowing above the water, and told me he had sailed over that 

 spot, and that in a few years there would l^e dry land there. 



'* The mode of growth of this low-lying land, then, has been as 

 follows: — 



•' (The shallows of the Passage are covered with lightly rooted 

 marine grasses and weeds.) 



-' Back-waters or cross currents pile up a bank or shallow, the 

 heavy mangrove seeds settle and take root. The mangrove once 

 having taken root, not only puts its branches above the water 

 but its roots above the soil. Anyone wdio has walked under a 

 mansfrove tree will remember that for yards round its trunk 

 there are hundreds of spikes, a few inches long, sticking up from 

 the soil in which it is growing. 



" Here, then, we have a natural rake; the numbers of closely 

 placed trees stop all that floats on the surface, while their roots 

 stop heavier rubbish (loosened w^eeds) and sand moving along the 

 bottom. Once our embryonic island reaches high- water level the 

 rank scrasses of the district take a hold and do their share of 

 raising its height. Masses of matted grass roots, retaining soil, 

 eighteen inches to two feet in thickness are frequentl}' met with 

 on the beach outside the northern end of Bribie Passage." 



viii. Conclusion. 



The present work was undertaken partly on account of the 

 great lack of detinite knowledge as to the geology of the Glass 

 House Mountains, the views of different authorities varying 

 within such wide limits; and partly to try to assist in the task 

 of investigating and correlating the Australian trachytes. As 

 shown in the part dealing with "Other Australian Trach^^tes," 

 the work has been energetically tackled in Tasmania, Victoria 

 and New South Wales by able investigators, whereas the Queens- 

 land trachyte areas have remained for the most part untouched, 

 though of equally great importance and interest. 



I am fully aware of the ditJiculty of the task I have undertaken. 

 In a preliminary paper like the present it is impossible to deal 

 with the subject so thoroughly as could be desired. Many 



