ON THE ROCKS OF THE HAURAKI GOLD-FIELDS. 247 



the general tint is dark grey with bright specks ; a few microlites 

 can be seen and more abundant anisotropic angular grains without 

 polarization colours. With an eightli objective it appears as a 

 water-clear glass with minute rods and specks, and occasional layers 

 of larger, irregularly shaped, dark fragments. The quartz-sand- 

 stone of Mr. Davis has a microscopic structure similar to that of 

 the felsite, but has in addition quartz grains scattered through it 

 as v,-ell as abundant pyrites. The specific gravity of the felsite 

 without pyrites is between 2.494 and 2.505, and Mr. W. Skeyhas 

 made the following partial analysis of it :— 



Silica 73.46 



Alumina 22.11 



Lime ... ... ... .77 



Magnesia ... ... ... 1.34 



Alkalies ... ... ... 1'56 



Water ... ... ... .76 



100.00 

 Mr. Cox has observed a similar rock interstratified with the 

 slates of Coromandel {he. cil. p. 7), and I am now convinced that 

 this felsite is of clastic origin. It resembles a felstone in appearance, 

 but is much softer and has an earthy fracture, while the small 

 quantity of water contained in it forbids it being considered a 

 slate or clay-stone. Perhaps it will be better to return to the 

 name felsite-tuff, which I originally gave it. 



POST-JURASSIC ROCKS. 



These cover the greater part of the Peninsula, and are almost 

 entirely volcanic. At Coromandel and at Kennedy's Bay, dis- 

 tinctly stratified scoriaceous agglomerates are found which are the 

 youngest rocks of the formation ; but elsewhei'e, so far as my 

 observations go, there are no traces of stratification, no vesicular 

 rocks, and difterent laA'a streams can rarely be distinguished. 

 A'^olcanic breccias frequently occur, but they usually pass imper- 

 ceptibly into unbrecciated rock. Hard dark rocks are comparatively 

 rare, usually they are light-coloured — grey or greenish — and with 

 a trachytic habitus, but soft. At Coromandel they were called 

 trachytes by Dr. v. Hochstetter, and this has apparently been 

 confirmed by Mi\ W, Skey, who found the alkali in them to be 

 potash.* However, both at the Thames and at Coromandel, I have 

 always found the felspars to be plagioclase, probably labradorite, 

 or a still more basic variety. 



When I first examined the district, twenty years ago, I was 

 much puzzled with these volcanic rocks, for they Avere unlike 

 anything I had sesn before, indeed their equivalents are found 



♦Report on Geology of Thames Goldfields, 1867, pp. 5-6. 



