BY H. I. JENSEN. 165 



felspar before decomposition seems to have been chiefly orthoclase; 

 hence this rock is a trachyte tuff. The phenocrysts are broken, 

 and have their long axes lying chiefly in the same direction as 

 the finer constituents of the rock. Hence they were deposited 

 under water, and here we have petrological evidence verifying field 

 evidence to the same effect. Most of the tuffs of the Maroochy 

 district are bedded, and sometimes contain intercalated shaly 

 bands. The groundmass consists of dumbbell-shaped, boomerang- 

 shaped, comb-shaped, and other fragments, most of them 

 with concave outlines. The rock has thus the typical structure 

 of an acid tuff. 



Relationship and Succession of the Vol- 

 canic and Hypabyssal Rocks — The gradation of 

 rhj'olite into trachy-comendite in the Maroochy district may 

 be taken as evidence of the close relationship and contem- 

 poraneity of these rocks. At Mt. Archer too we have rhyolites 

 and orthoclase porphyries of the same age. In the Glass House 

 Mountains we have good evidence, in the form of trachyte inclu- 

 sions in dacite, that the dacites were erupted later than the 

 trachytes In the Maroochy district there is no evidence that 

 the dacites are later, though the order observed in the Glass 

 House Mountains probably holds here too. The Mt. Cooroy 

 monzonite is closely allied to some of the quartz andesite dykes 

 near Eumundi, and between Yandina and Nambour. Tt has 

 also, as already shown, affinities with the Noosa and Pt. Ark- 

 wright mass; its age is hard to arrive at; it is probably older 

 than the trachy-rhyolites. The quartz andesites and dacites are 

 divisible into two kinds: (a) those lithologically resembling the 

 Glass House Mountain dacites; they occur at the Nindherry 

 Saddle (Plate v.) and around Mt. Wappa, and are partly glassy; 

 these are probably subsequent to the rhyolites. (b) Those which 

 are noncrystalline with trachytic texture, and closely allied to 

 the Cooroy rock in composition. They are intruded by rhyolite 

 dykes and are therefore earlier than the rhyolites. The Pt. Ark- 

 wright rock and the Noosa Hd. Quartz- Diorite are both closely 

 allied in basicity to the earlier andesites, and the fact that the 



