OF THE ROSS ARCHIPELAGO 



135 



clase on the other hand exhibiting the fine cross-hatching typical of the mineral as well 

 as sometimes only fine albite twinning (Fig. 2, PI. I). Both varieties have refractive 

 indices less than those of Canada balsam and clove oil (n= 1536) but when fragments 

 are immersed in monochlor- benzene (n= 1*525) the indices of the sanidine are mostly 

 below that of the liquid, 7 alone being of about the same amount, while those of the 

 anorthoclase are mostly above 1 "525, a alone being about the same. In habit the anortho- 

 clase appears to form stouter and shorter prisms. 



The following table gives the mineralogical composition observed by crushing 

 selected fragments of the rocks in suitable liquids : 



So fragile are the rocks that the sections obtained do not give much further informa- 

 tion about the structure. The felspars when lying free in glass have perfect crystal 

 outlines (Fig. 2, PI. I), but most often partially interfere with one another. Opaque 

 inclusions are somewhat abundant and are generally scattered irregularly through the 

 mineral, but occasionally lie in vertical planes in the crystal. The aegirine-augite is a 

 strongly coloured yellow-green variety with a slight pleochroism. Cleavage flakes 

 give extinction angles of 37°-39°, whilst the largest extinction angle measured in section 

 (not quite parallel to 010) is 43°. In part, at least, the mineral has crystallised after 

 the felspar, on which it is moulded (Fig. 3, PI. I). Fig. 4, PI. I shows a crystal of 

 a?girine- augite lying in a druse of the rocks partly filled with ground-mass. The olivine, 

 though abundant in hand- specimen in well-formed crystals, is poorly represented in 

 section. It occurs frequently in the trachyte in the neighbourhood of the inclusions, and 



