PETROLOGICAL NOTES ON FURTHER ROCK SPECIMENS. 319 



The presence of some few scales of calcite afford evidence that dynamic effects 

 have been more potent in the alteration of the rock than heat, though, of course, the 

 part played by the latter factor has been by no means inconsiderable. 



No. 974. Hornblende-Plagioclase-Pyroxene Gneiss. 



This rock, like No. 942, which was described by Stillwell, occurs in dyke-like bands 

 up to 10 feet wide crossing the garnet gneiss. The rock is dark, dense, and fine-grained, 

 although the granular nature is differential, in that there are fine-grained portions 

 surrounded by more coarsely crystalline rock. One face of the hand specimen shows 

 a remarkable development of pale brass-yellow sulphide ore, probably pyrrhotite. 

 Of the faces which were exposed to weathering, one has the typically indurated 

 appearance common to the rocks of this area, and another has been coloured yellow 

 by the formation of some limonite by oxidation and hydration. 



The rock may have been formed in the manner Stillwell suggests for No. 942, but 

 the presence of an occasional crystal of pink garnet shows that the metamorphism 

 suffered by the rock has been a little more intense, or that there has been a slight 

 variation in the composition of the rock. 



The rock, like No. 972, contains some hypersthene as well as monoclinic pyroxene, 

 calcite, well formed grains of magnetite and some crystals of apatite. An occasional 

 pleochroic halo, with zircon as-nucleus, is found in the biotite. 



No. 977. Garnet-Plagioclase Amphibolite. 



This is a moderately medium-grained, massive rock in which the most prominent 

 minerals are red garnet and biotite. Hornblende is quite abundant and felspathic areas 

 are readily observable. 



The most striking feature of the rock is the presence of remarkable skeletal 

 crystals of ilmenite whose peripheries are composed of many curved hooks or fingers 

 which come from the main mass. Their relationships with the green hornblende of the 

 rock is made manifest by the reaction zones which exist between the two. These reaction 

 zones consist of a dactylitic intergrowth of felspar and pyroxene called diablastic by 

 Stillwell, but the term used here is preferable as it emphasises the finger-like nature of the 

 components which enter into the intergrowth. The ilmenite is the nuclear region where 

 concentration of iron (and titanium) from the hornblende is occurring : the dactylitic 

 region shows where the actual diffusion of matter can be regarded as occurring. The 

 source and the goal of the transfer are at once made evident. 



Occasionally crystals of garnet are set in a rim of clear felspar which separates 

 it from hornblende. In the felspar are to be seen very minute, but well shaped, 

 rectangles of pale green amphibole which seem, in part, at least, to play a similar part 



