Speight. — Collection of Rocks from X or folk Island. 329 



up to 2 cm. in diameter imperfectly cemented together. The higher tuff 

 is much finer in texture, and of yellowish-brown colour. These rocks are 

 so friable that it is impossible to make sections from them, but a micro- 

 scopical examination of fragments shows that they are composed of 

 essentially the same material as the overlying rocks, and grains of olivine, 

 decomposed augite, and feldspar form the bulk of their substance. Be- 

 tween the upper and lower portions oi these tuffs is a variety, largely 

 composed of volcanic material, greenish-grey iu colour, soapy to the feel, 

 but microscopically presenting no marked features to differentiate it from 

 the overlying beds. 



The uppermost member of the series in this locality consists of a dark- 

 grey moderately coarse-grained rock in which the feldspars and olivine 

 grains are clearly visible to the eye. It is even-grained in texture and 

 slightly vesicular ; its specific gravity is 2-60. 



The microscope shows that it is composed of broad lath-shaped feld- 

 spars (med. labradorite), which are usually perfectly clear and unaltered, 

 and frequently enclosed ophitically by crystals of faintly coloured augite. 

 The oli\dne3 are numerous in large grains, 14 mm. in diameter, often 

 arranged in aggregates, with many smaller pieces scattered through the 

 section ; they are occasionally clear, but usually stained with iron oxide. 

 There is a considerable quantity of titaniferous magnetite in grains and 

 broken-comb shaped aggregates ; some of this is certainly secondary. The 

 rock is therefore a dolerite of distinctly basic character. 



DuNCOMBE Bay, North Side of the Island. 



At Buncombe Bay, on the north of the island, tuffs of similar 

 character occur, overlaid by both fine-grained and coarse-grained basic 

 rocks. The tuffs are similar in mineral composition to those at Anson 

 Bay, the lower beds being yellow in tint, while the upjjer beds are distinctly 

 red. 



The rock immediately overlying the tuffs is a dolerite of somewhat open 

 texture, grey in colour, with abundant olivine showing in the hand-speci- 

 men ; its specific gravity is 2-74. In section it appears to be principally 

 composed of plagioclase (basic labradorite) in broad laths, exhibiting at 

 times a rude parallel arrangement ; augite in grains 0-5 mm. in length, 

 much altered ; and olivine in grains up to 0-6 mm. in diameter, occasionally 

 fresh and clear, but usually stained brown with iron oxide round the 

 edges or in cracks ; a considerable amount of magnetite is also present. 

 Associated with this is a finer-grained type of similar mineral composition. 

 No glass is present, but the olivine phenocrysts are exceptionally promi- 

 nent. The specific gravity of this rock is 2-87. 



Cascades, North-east Side of the Island. 



The collection from this locality includes specimen's of colunmar basalt, 

 a coarse-grained basalt, a fine-grained basalt, and a reddish vesicular rock 

 from a dyke. The specific gravity of the specimen from the boulder 

 is 2-74. In section these rocks are all of the same general type, with vary- 

 ing degrees of coarseness. The columnar basalt and the specimen trom the 

 boulder are remarkably fresh rocks, the freshest m the collection. The 

 groundmass appears in section to be composed of laths of plagioclase 

 (labradorite) with fluxion arrangement, numerous grains of augite, much 

 magnetite with rectangular outlines. The phenocrysts are exclusively of 

 fresh olivine ; occasionally the crystals show idiomorphic outlines, but 



