BY E. C. ANDREWS. 161 



Petrological Notes. 



The igneous rocks consist mostly of granites of acid types, quartz 

 and quartz-felspar porphyries, both Palaeozoic in age, and intruded 

 alike by regular networks of basaltic dykes. 



Granites. — These have a marvellous development, and are 

 principally of acid types. Many of the islands, as also the high 

 coastal ranges, are composed entirely of these plutonic rocks. 

 They are probably Carboniferous and Permo-Carboniferous in age. 



Quartz and felspar are the chief constituents, although horn 

 blende and biotite are frequently present. In some of the island 

 groups, such as the Palms, the rocks may consist of quartz and fel- 

 spar, with idiomorphic hornblende and biotite flakes. The horn- 

 blende, which is strongly pleochroic, occurs in hexagonal, rhombic 

 and rectangular forms in thin sections, and post-dates the formation 

 of the biotite. Again, hornblende may be absent, while in other 

 cases flat veins of aplite with ill-defined boundaries occur; they 

 are composed mainly of micrographic growths of quartz and 

 felspar. Large segregations of orthoclase and quartz may be 

 seen in the rock exposures, and epidote at times replaces the 

 ferromagnesian constituents. Zircons are associated with the 

 hornblendic varieties. Here, also, crushing is a marked feature. 

 Under the microscope the quartzes and felspars are seen to have 

 undergone great peripheral crushing, besides wliolesale fracturing 

 of the different minerals constituting the rock. The grains of 

 quartz and felspar appear as if cemented in a base of the same 

 material (mortar structure). Splendid examples of undulose 

 extinction are shown by both quartz and felspar crystals; long 

 lines of liquid inclusions occur in the quartz, while the felspars 

 (orthoclase and acid plagioclase) show zonary structure, and the 

 development of fresh minerals along solution planes. Biotite also 

 occurs in bent flakes, and secondary quartz is present in great 

 abundance. 



Other granites, collected from Fitzroy and Goold Islands 

 although altered, do not show the effects of metamorphism in so 

 marked a desfree as the Palm Islands varieties. 



