146 VOLCANIC ROCKS OF SOUTH-EASTERN QUEENSLAND 



there is a hardened band up to 2 feet thick right at the 

 base of the tuff. This band is more compact than the 

 overljdng material and in many respects it suggests a 

 rhyolite flow. It is noticeable, however, that this band is 

 best developed where the overlying tuff is most silicified, 

 and it is highly probable that it represents a layer of tuff 

 at the bottom which has received an extra amount of 

 silicification owing to the percolating solutions being held 

 up by the impervious underlying shale band. 



The greatest development of acid pyroclastic material 

 occurs along ]\IacPherson 's Range and the ridges running 

 off it in a northerly direction. It is distributed over an 

 area of about 300 square miles, and was in all probability 

 almost continuous. In general appearance, it is remark- 

 ably uniform as far as the matrix is concerned, but there 

 is considerable variation in the size and abundance of the 

 included fragments. 



Occurrences of tuff are only occasional and the most 

 typical development showing definite bedded structure was 

 met with on the northern banks of the Coomera River, a 

 few miles M^est of the Gin's Leap. The tuff has in places 

 been much silicified and made very compact, although it 

 is usually in a rather loose state of aggregation. The matrix 

 is usually very fine-grained and a dirty yellow in colour, 

 though it may be white or a bright green in patches. The 

 greenish colour seems to be due to the presence of chloritie 

 material. 



The included fragments consist for the most part of 

 rhyolite, but near the base there are large rounded boulders 

 of basalt very similar to the underlying basaltic flows. The 

 rhyolite inclusions range from holocrystalline and porphy- 

 ritic varieties to pitchstones, and a black glassy pitchstone 

 is a very widespread and characteristic inclusion. These 

 inclusions often show a definite breeciated or welded 

 structure. 



On portion 136, parish of Telemon, at Mr. P. Burnett's 

 selection on Chinghee Creek, a silicified tree trunk about 

 12 inches in diameter and 4 feet long was found included 

 in the agglomerate. Thin veins of silicified ash material 

 were seen along what had apparently been cracks in the 



