168 GEOLOGY OF QUEENSLAND COAST, 



Porphyries. — These consist of quartz and felspar idiomorpbs 

 scattered plentifully through a fine granular base of the same 

 minerals. The quartz and felspar may be porph3^ritic towards 

 the rest of the rock. Mica plates are common. 



Felspar occurs as stout orthoclase and plagioclase crj'stals, and 

 much altered. The quartz contains abundant liquid inclusions. 

 Some of the crystals are much corroded, the magma having almost 

 penetrated to the crystal centres from every direction. 



The base is rarely felsophyric with abundant magnetite in 

 grains. 



Dolerites and Basalts. — Throughout the acid rocks just men- 

 tioned basic dykes occur in great numbers, so as to appear at 

 times in the cliff faces as regular networks of veins. 



The rocks are both holocrystalline and hypocrystalline, and 

 consist principally of plagioclase laths and augite grains. Olivine 

 is frequently present. Hornblende also occurs in certain types, 

 but much decomposed. The olivine is principally decomposed. 



Ophitic structure is noticeable in certain sections. 



These basic intrusions belong probably to the Tertiary period. 



Mr. Jukes and Professor Agassiz spent some considerable time 

 on the Great Barrier Reef. Saville Kent also studied Barrier Reef 

 problems on the spot for a period of twelve months. Jukes 

 furnishes a section (reproduced by Agassiz, I.e., p. 137) across the 

 Barrier Reef and to the mainland which sums up his ideas of its 

 origin, viz., that (even allowing for his exaggerated vertical scale), 

 generally speaking, the continental shelf from the coastal plains 

 of the mainland to the outer Barrier is composed of reef and reef 

 debris. Notwithstanding this, his description is at variance with 

 his section (postea, pp. 170-172). Jukes also sa3^s* that the Cxreat 

 Barrier Reef, longitudinally considered, " would be found to ha\e 

 a considerable resemblance to a gigantic and irregular fortification, 

 a steep glacis crowned with a broken parapet wall, and carried 

 from one rising ground [Sir C. Hardy's Islands] to another" 

 [Breaksea Spit]. 



* Jukes, Voyage of H.M.S. "Fly," Vol. i., p. 332-333. 



