180 VOLCAXIC ROCKS OF SOTJTII-EASTERN QUEENSLAND 



representatives of all parts of the area, and it is notable 

 that there are no representatives, belonging to this division, 

 of the definitely alkaline series. The author is of the 

 opinion that a combination of the analyses in equal pro- 

 portions furnishes the most reliable basis for a determina- 

 tion of the mean average composition of the lavas. The 

 analyses used were those of lavas numbered 18, 20, 22, 23, 

 24, 25, 26. 27, 28, 29, 31, 32, 33, 34. 



Average Composition of the Volcanic Eocks of the 

 Middle Division. 



This estimation is rather more complex than the above, 

 and seventeen analyses were used. 



The rhyolitic rocks in the southern part of the area, 

 the rocks of the Esk district, and the alkaline rocks from 

 the Glass House Mountains, Mount Flinders and Main 

 Range areas, constitute the representatives of this division. 



It is estimated that rocks of this division in the Main 

 Range, the Esk, the Glass House ^Mountains and the 

 Mount Flinders areas are of nearly ecjual importance. 

 Consequently the analysis of the soda trachyte from the 

 ]Main Range was combined equally with each of the three 

 averages derived from (a) the five Esk rocks, (6) the four 

 Glass House Mountains rocks, and (c) the three IMount 

 Flinders rocks. 



The above rocks together total approximately one 

 quarter of the extensive development of middle division 

 rocks in the southern and south-eastern portions of the 

 areas. 



The Chinghee Creek, IMount Lindsay, Mount Barney, 

 and Springlu'ook rocks are all very similar and are repre- 

 sentative of the southern and south-eastern material. 



Consequently the average obtained by taking each of 

 these latter four rocks and the above average in equal 

 proportions is regarded as being approximately represen- 

 tative of the extruded material of the middle division. 



The analyses used were those of the rocks numbered 

 1, 2. 3. 4. 5, 6,^7, 8. 9, 10, 11, 12. 14, 16, 17, 19, 21. 



