Vol. 70.] OF THE COUKTBY ABOUND IIUXTLY. 285 



and the dominant ferromagnesian mineral is biotite, forming inter- 

 growths in the less advanced stages. Myrrnekitic intergrowths of 

 the felspars and quartz are less common, and slight decomposition 

 may appear. Garnets of both kinds are found often with recti- 

 linear boundaries, and sometimes associated with brownish-green 

 spinel. Of the felspars, orthoclase and labradorite are certainly 

 present, and in some cases the plagioclase has the acidity of oligo- 

 clase. These rocks have a specific gravity of 304. 



In the above-described rocks, silliinanite appeared in the usual 

 fibrous form, but a further stage is exemplified by the appearance 

 of the platy variety. The plates of the mineral are in this case 

 surrounded by a ring of green pleonastes and grains of cordierite. 

 Hypersthene has disappeared, and the biotite shows intergrowths 

 with quartz. 



Under slightly different conditions it would seem that a hyper- 

 sthene is again formed, as in the rock from above Roddentree. 

 Here the minerals now present are garnet, hypersthene, cordierite, 

 and biotite. The hypersthene is spongy, containing numerous 

 inclusions of quartz and felspar. The felspars are basic and acid 

 plagioclase, as well as orthoclase, which is frequently ringed round 

 by a quartz-felspar-rnicropegmatite, similar to the structure seen in 

 acid plutonic rocks. It is, of course, possible that this represents a 

 type at the beginning of the change from hypersthene to biotite, 

 but the hypersthene of the norite is remarkably free from inclusions 

 such as those mentioned above. In fact, this hypersthene strongly 

 resembles that found in the contact-rock at Kothiemay mentioned 

 previously (p. 280), and a spongy hypersthene is usually one of 

 secondary origin. From its field-relations, it is difficult to see how 

 this rock could represent a contact -altered sediment. 



A somewhat different type of development is that seen in the 

 xenolith of the Truttle Stones, where the contact-rock is composed 

 almost entirely of biotite and cordierite. To these may be added 

 labradorite and pink garnet. The halos in the cordierite are 

 numerous and very fine, being occasionally double. 



The pegmatite-veins from the Avochie Granite have produced 

 near Ladysmith rocks meriting the name of cordierite -norite. 

 The hypersthene occurs in large crystals, but has been invariably 

 attacked to form other minerals. Throughout the rock the other 

 minerals occur in small grains. From the hypersthene are developed 

 pleonaste, magnetite, and colourless garnet. The pleonaste and 

 magnetite may occur as inclusions in the hypersthene, in which case 

 they are separated from the pyroxene by a thin band of mineral — 

 usually colourless garnet, but also apparently cordierite. The 

 colourless garnet possesses vermicular markings interlocking with 

 similar projections from the spinel, where the two minerals are in 

 contact. The hypersthene is here broken up, and the colourless 

 rings indicate the medium by which the transference of material 

 was effected. The spinels reach the comparatively large size of 

 0*53 mm. Cordierite, felspar, and pleonaste make up considerable 

 parts of the rock. 



