Vol. 62.] GEOLOGY OF DT/NEDIN (NEW ZEALAND). 391 



rocks ; and the examination and classification of these furnish a 

 considerable aid in fixing the relative ages of some of the different 

 rock-types. Their consideration will be left over, until the discussion 

 of the various rock-types to which the boulders belong. Wherever 

 these conglomerates have been observed, they are covered by sheets 

 of lava, usually of a basic character. 



Alluvium of recent accumulation fills the lower portion 

 of many of the stream-valleys. It consists usually of gravel. Its 

 presence is of little interest or importance, except as showing that 

 some depression has taken place since the valleys were eroded. 



Blown sand is found in all the sheltered bays round the coast. 

 The sand is first deposited on the beach, whence it is blown until 

 its further course is arrested by the growth of vegetation or by the 

 opposition of some rock-mass. The sand has been deposited in 

 greatest quantity on the northern and north-western sides of the 

 inlets, especially those larger ones that have prominent projecting 

 south-eastern points. 



In most instances the rapid growth of vegetation has prevented 

 the sand from blowing on to the adjacent hill-slopes. At Sandfly 

 Bay and at Bauone Beach, near Taiaroa Head, the growth has not 

 been sufficiently rapid to prevent this movement, and the sand has 

 been blown over a saddle 800 feet high at Sandy Mount and 400 feet 

 high at Bauone. 



This sand is formed almost exclusively of quartz-grains. In 

 localities where the prevalent south-westerly and north-easterly 

 winds have full play, the backward and forward movement of the 

 sand has produced dreikanter with a sharp summit- ridge directed 

 from north-west to south-east. 



Igneous Rocks. 



1. Hornblende-foyaite. — The only rock classed under this 

 head occurs apparently as a massive intrusion, at the base of the 

 Harbour Cone near the shore of Hooper's Inlet. It is a light- grey 

 type, showing felspars and amphibole in the hand-specimen, with 

 some pyrite which is most frequent round the border of the horn- 

 blende-crystals. 



In section, the rock shows an allotriomorphic structure. Some of 

 the felspar is orthoclase and part oligoclase-andesine, twinned after 

 the albite and sometimes after the pericline-law. A little nepheline, 

 much decomposed, is also present. The hornblende is dark brown, 

 with the pleochroism of arfvedsonite. It is surrounded by a 

 broad resorption-ring, consisting of segirine-augite in very fine 

 grains, magnetite, and pyrite. The hornblende is penetrated by 

 numerous, short, stout prisms of apatite. (See PI. XXXVII, fig. 1.) 



Chemically, this rock agrees closely with foyaites from other 

 localities, as the following analyses demonstrate : — 



