AMPHIBOL1TES AND RELATED K<>rK- -Til. I. \VKLL 275 



IV.-GROUP III.-THE PLAGIOCLASE GNEIS8E8. 



With a decreased percentage of hornblende and an increased percentage of 

 felspar, the amphibolite types pass into less basic rocks which correspond to the 

 plagioclase gneisses of Group III. In many specimens quartz becomes a more important 

 constituent and a considerable variety of rocks, including some transitional varieties 

 between Groups I and IV. are placed hen-. 



KATA DIVISION. 

 Xone of the specimens examined appear to fall into this group. 



MESO DIVISION. 



Among the hornblende-plagioclase-gneisses are specimens Nos. 169, 170, which 

 are light-coloured, coarse-grained rocks, in which the hornblende appears as small 

 schlieren in the light-coloured base, and forms about 16 per cent, of the rock. The 

 base contains quartz and alkali felspar in addition to the plagioclase. These specimens 

 also contain fragments of much larger schlieren composed of dark amphibolite or green 

 pyroxene. In the proximity of the amphibolite schlieren there is a band containing 

 epidote, ealcite and lawsonite along which the hornblende is partially altered. The 

 alteration (PI. XL, fig. 4) takes place first to a colourless hornblende and then into an 

 aggregate of epidote, lawsonite and calcite. The pyroxene in the pyroxenic schliere 

 shows alteration to hornblende, and there is in addition a separation of calcite when 

 fragments of pyroxene are set in a network of calcite. 



Specimen No. 378 is a pink hornblende-felspar-gneiss, whose colour is due to 

 the pink felspar modified by the dark crystals of hornblende and mica. In section, 

 it has a granoblastic structure and a schistose texture. The colourless constituents 

 include quart/, clear microcline, plagioclase with accessory apatite. The dark 

 constituents are green hornblende, greenish-brown biotite, which is partly altered 

 lu chlorite and epidote, while split-lie is an abundant accessory mineral. The rock 

 may be called a hornblende-felspar-gneiss, and on account of its content of quartz 

 and alkali felspar it probably has a chemical composition grading towards that of 

 Group I, the alkali felspar gneisses. 



No. 269 is a hornblende-plagioclase-gneiss, which is more basic than No. 378, 

 and probably grades in the opposite direction towards the group of amphibotites. No. 

 378 contains pink felspar in the hand specimen, but No. 259 possesses more hornblende 

 and white felspar. No. 259 also possesses a coarser grain size, and some of the 

 hornblende crystals are 4mm. wide. In section, the large hornblende crystals are 

 green and bluish-green in colour, ragged in outline, and frequently contain inclusions 

 of biotite. Some are partially broken across the biotite inclusions, and epidote haa 

 developed in places along the strained portions of the hornblende. Biotite is very 



66-2. 



