112 VOLCANIC ROCKS OP SOUTH-EASTERN QUEENSLAND 



while at Currumbin and Burleigh Heads there are ridges 

 of Palaeozoic- rocks, in some cases capped by basaltic rocks, 

 dividing the plain. 



The drainage of the various plateaux in the south 

 usually descends over precipitous cliff's, often 500 feet or 

 more in height. These falls are very frequent, and the 

 head of each creek generally has its falls several hundreds 

 of feet high. The course of the stream is then usually 

 through a caiiyon-like valley with precipitous cliffs, in some 

 cases 2,000 feet high, on either side ; and the valley itself 

 may be only half a mile wide. This young valley usually 

 merges suddenly into a mature and well-rounded valley 

 immediately the plateau is traversed and the Mesozoic 

 sediments without a capping of volcanic rocks are entered 

 upon. The contrast between the valleys dissecting the 

 plateaux of volcanic rocks, and the valleys in the denuded 

 ]\Iesozoic areas, is very pronounced, and is, no doubt, almost 

 entirely due to the differential rates at which the rocks are 

 worn down. 



The streams flowing over the JNIesozoic portion of the 

 area are very similar in nature, and their maturity is very 

 evident. If one views a valley such as the Reynolds' Creek 

 Valley from ]\Iount Greville or the Canungra Creek Valley 

 from Tamborine Plateau, the serpentine-like line made by 

 the dark vegetation which grows on the banks of the streams 

 is very definite. The average fall of the streams when once 

 they have left the plateau area is approximately 5 feet per 

 mile. 



The steep cliffs which fre(iuently form impassable pre- 

 cipices around the plateaux are due in many eases to the 

 undercutting of the loosely-aggregated acid volcanic 

 agglomerate and tuff. This acid fragmental rock is usually 

 near the lower level of the cliffs, and owing to its ease in 

 wearing away, it leaves no support for the overlying hard 

 basic \'olcanic rocks which consequently fall away leaving 

 steep inaccessible cliffs. 



All of the plateaux are of the same structure. They 

 are made up of a lower series of sub-basic and basic rocks 

 resting on either Palaeozoic or Mesozoic sediments; on top 

 of this, one finds either acid tuff, agglomerate, or lava, and 



