OF THE ROSS ARCHIPELAGO 133 



augite, on the other hand, is seldom free from ferruginous inclusions, both magnetite 

 and limonite are sometimes scattered irregularly throughout the mineral, and some- 

 times so disposed in small rods as to produce a schiller structure parallel to the vertical 

 axis, while rows of negative crystals occasionally run parallel to the base. Hypersthene 

 and enstatite show a similar schiller structure in a plane parallel to the vertical axis. 

 There appears to be no difference between the augite of the pyroxene and gabbroid 

 nodules, both giving extinction angles of over 40°. The felspar is a very basic variety 

 with extinction angles, on symmetrically cut albite lamella?, as high as 37° ( labradorite- 

 bytownite). 



As regards the quantitative relationships, in the olivine nodules olivine predominates 

 greatly over the other ferromagnesian minerals, which are generally present in quite 

 subordinate amount. In a few cases, however, augite occurs in such abundance that 

 the rocks may well be termed pyroxene nodules. The rhombic pyroxenes are never pre- 

 sent in large quantity, a feature which Lacroix has noted as common when hornblende 

 is present.* In the gabbroid nodules the ratio between felspar and ferromagnesians 

 varies somewhat widely, as also does that between augite and hornblende. It will be 

 noticed that the spinel of the olivine nodules is chromite, whereas that of the gabbroid 

 types is pleonaste. Mr. W. N. Benson, of Sydney University, has pointed out to 

 me that the same law of association prevails in the inclusions of a large neck at 

 Dundass, near Sydney. 



The structural relations call for little mention. Most often the rocks are quite 

 massive, but in one specimen (1909)f there is a well-marked banding of felspathic and 

 ferromagnesian minerals. The olivine nodules are typically granular rocks, neither 

 olivine nor augite showing definite crystalline form (Fig. 1, PI. I). Hornblende and 

 biotite, if present, are always the last elements of consolidation. In one specimen 

 (1193) there is a small veinlet of brown hornblende traversing the nodule, recalling 

 on a minute scale the association of hornblendites with the lherzolites of the Pyrenees, f 

 In the gabbroid nodules also the order of crystallisation is not well marked, although 

 olivine is clearly anterior to augite. There is no suggestion of ophitic relation of pyroxene 

 to felspar, but sometimes the augite appears to be intergrown with the felspar on its 

 edges. The brown hornblende surrounds the augite, and in one case is found chiefly 

 between the plagioclase and the olivine, probably in consequence of a magmatic resorp- 

 tion of the latter, but there is no radial orientation of the hornblende as in the well-known 

 reaction rims, nor is the hornblende found in parallel position on the augite. 



Although hornblende is a frequent constituent of the above inclusions, it does not 

 appear as the main constituent of hornblende nodules, such as are often found in horn- 

 blende basalts. T he collection includes, however, one large cleavage fragment of brown 

 hornblende measuring 3 by 1/5 cm. in its greatest diameters. It shows in places a 

 rounding and polishing that may be ascribed to magmatic corrosion. 



As regards the origin of these inclusions, there is no doubt that they are closely 



* hoc. cit., p. 48G. 



f The numbers attached to the specimens in this paper require some explanation. When the 

 paper was written, in September 1910, many rock specimens collected by the Expedition had not 

 been given registered numbers, and microscopic slides made from such specimens were labelled 

 T. 1 2, etc., when made to my order, P. 1, 2, etc., when made by Mr. Priestley, and E. 1, 2, etc., 

 when belonging to the general collection of slides. I understand that the specimens of which 

 slides were prepared were deposited, with others, in the Museum of the Geological Department of 

 Sydney University, but that a duplicate collection was sent to Professor Lacroix at the Musee 

 d'Historie naturelle, Paris. — J. A. T., January 1916. 



j Lacroix, A., " Les roches basiques accompagnant les lherzolites et les ophites des Pyrenees," 

 Compt. Rend, viii, Cong. Geol. Inter., Paris, 1900, p. 806. 



