ORIGIN OF CERTAIN CONTACT ROCKS. 179 



crystallisation for the group is vesuvianite, diopside, phlogopite, 

 chlorite, serpentine. 



B. — Axinite-Actinolite-C alette and Garnet-V esuvianite-Actitw- 

 lite Rocks. — ^The members of this group are best developed in 

 Tasmania in two parts of the North Dundas district. The principal 

 occurrence, and that which has best been exposed by mining 

 operations, is that on the Colebrook Hill. A number of smaller 

 developments are known in the part of the North Dundas tinfield 

 which lies to the south-west of the Colebrook mine. With these 

 occurrences the following description is concerned. A related 

 occurrence would seem to be that of the Mt. Ramsay bismuth mine, 

 in which the non-metallic gangue minerals are amphibole and 

 fluoritei. In this material axinite was detected by the late Pro- 

 fessor Ulrich. This occurrence has not been investigated officially 

 by the geological survey, but the specimens obtained from it display 

 undoubted genetic affinities with the rock of the Colebrook Hill. 



(a). Axinite-Actinohte-Calcite Rock (Limurite).— The speci- 

 mens to be obtained from the Colebrook mine show no essential 

 features of difference from those of the Boulder mine, and the 

 following description appHes to both. 



The rock is of very variable appearance and is commonly 

 banded. The colour is usually violet or greenish, according to 

 the predominance of axinite or actinolite, but is sometimes green 

 and white when calcite carrying bands of actinolite is abundant. 

 Bands of slate are often to be seen in the mass of the rock. Quartz 

 may often be recognised, and, rarely, danburite and datoHte. The 

 grain is very variable, and so are the proportions of the component 

 minerals. 



The metallic minerals characteristic of the group are pyrrhotite, 

 pyrite, arsenopyrite, chalcopyrite, with minor amounts of galena, 

 blende, and tetrahedrite. Bismuth has also been detected in small 

 quantities. These metallic minerals are either disseminated 

 through the mass or are restricted to bands or lenses, or, again, they 

 may entirely take the place of the non-metallic minerals which 

 belong to the same composite whole. 



The microscopical characters of the non-metallic portions are, 

 as may be expected, very variable in different specimens. The 

 axinite is always in idiomorphic or hypidiomorphic plates, which 

 are often large. It shows the same undulose extinction as has been 

 noted by Prof. Lacroix for the axinite of Arbizon and Montfaucon.2 

 Actinolite appears in somewhat variable forms — stellate aggregates, 

 sheaves, or separate acicular crystals, and in some cases forming 

 more massive crystals, of which the borders are corroded. Colour- 

 less pyroxene is common in small, stout idiomorphic prisms, of 

 which the outlines are at times corroded. Interstitial calcite and 

 quartz are common and may locally predominate. Nests of 

 radiating tufts of chlorite with negative elongation are fairly 

 common. 



1 Vide : W. F. Petterd, " Catalogue of the Minerals of Tasmania," 1910, pp. 17, 74 131 132 



2 Bull. Carte Geol. France, No. 71, p. 59. 



