Physiography, Geology.] SUBANTARCTIC ISLANDS OF NEW ZEALAND. 



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(2.) Trachytes. 

 Overlying the granite at Miisgrave Peninsula is a trachyte tuff, and above it 

 a trachyte in solid flows and also in dykes and sills penetrating the granite. Tra- 

 chyte dykes occur at Circular Head, running in a north-west and south-east direction 

 — i.e., directly toward Musgrave Peninsula, as the probable centre of disturbance 

 (fig. 11). As they penetrate both the granite and the gabbro, they are un- 

 doubtedly younger than both. The tuf! which f rests 'on the granite of Musgrave 

 Peninsula, on the eastern side of the isthmus, is white "in colour, varying in coarse- 

 ness, with numerous fragments of partially rounded grains of quartz and plates of 



Fig. 11. — Granite Rocks, with Dykes and Sills or Trachyte, Musgrave Peninsula. 



feldspar (albite-oligoclase). The rest of the rock is a cementing-medium of kaolin, 

 calcite, and other products. Where the rock is finer in grain, the quartz and feldspar 

 fragments occur more sparingly, and the bulk of the rock is a fine-grained mass of 

 irresoluble matter. It thus shows all the characters of a trachyte tuff largely derived 

 from the erosion of the granite similar to that found beneath it. The trachyte flows 

 and dykes which overlie and penetrate it are much decomposed, and stained with 

 limonite. The microscopic examination is somewhat unsatisfactory on that account, 

 but it appears to be a trachyte of distinctly alkaline type. The phenocrysts are 

 of sanidine, anorthoclase, and sparingly plagioclase. The ferro-magnesian minerals 

 are quite weathered, and are represented by aggregates of iron-oxides whose origin 



