• 386 Albert V. James: 



'JMie following are present in order of decreasing abundance : — 

 Andesine, quartz, ortlioclase, biotite, chlorite, sericite, niuscovite, 

 apatite, pyrite, magnetite, arsenopyrite, calcite and zircon. The 

 chlorite, sericite, pyrite, calcite, and arsenopyrite are secondary. 



The ejctinction angles of the carlsbad and the lamellar twins dis- 



. close the fact that the plagioclase is basic andesine. Zoned crystals 



are very abundant, and" the zones are seen to be more basic as the 



• centre is approached. Certain bands of the zoned felspars were 

 sericitized before the others, showing that felspars of that 



^composition were not so stable in the presence of sericitizing agents. 

 Generally sericitization took place from the centre outwards, i.e., 

 from the basic to the acid plagioclases. In the sections studied 

 sericite in its turn tends to be kaolinized. 



Sumiiiary. — The rock from its chemical and mineral composition 

 •and its physical properties is a slightly altered granodiorite. In 

 the hand it appears fresh, and shows no sign of weathering. 



In the field it undoubtedly appears to be linked to the Gelli- 

 bi-and mass, which lias been described by Dr. F. Stillwell as adam- 

 ellite (II). Both are 500 ft. high, and have the same mineral com- 

 position, and approximately the same chemical composition. It 

 differs from the Gellibrand stock in having a slightly higher 

 lime content, and slightly lowjer silica content. 



Dr. F. Stillwell found that the proportion of plagioclase to ortho- 



• clase was less than 2 :I in the Gellibrand stock, but the writer by 

 the Rosival method found the proportion distinctly more than 2-:l 

 in the Bulla stock, which is adjacent to it. Slight differences in 

 mineral and chemical composition are probably local, for the two 

 rocks are similar in all other important characters. 



Granitic Intrusions. — Near Hanging Valley is a granitic dyke 

 20 feet wide, intruding the hornfels. Near the southern edge of 

 the main granitic mass there are several small dykes of microgranite 

 intruding granodiorite, while north of Bulla bridge there is a 

 dyke of aplite and a small one of quartz, both in granodiorite. 



The dyke near Hanging Valley is evidently a tongue from the 

 main mass, but the microgranite, aplite and quartz tongues 

 appear to have been derived from the acid residue of the magma 

 :after the outside portion had cooled and hardened. The magma 

 evidently sloped the palaeozoic sediments so quietly that the dip 

 and the strike are not only unaltered up to the southern contact, 

 but are continued at the northern junction. The sediments to the 

 west also are undisturbed. 



