PROCEEDINGS OF SECTION C. 455 



Greenbushes stibiotantalite exhibits only occasional traces of an 

 external crystalline form : the only faces recognised with any cer- 

 tainty are a and b, with possibly m and u. A very distinct cleavage 

 parallel to the macropinacoid is almost invariablt noticed, and at times 

 traces of a second cleavage have been seen. In color and transparency 

 it varies very largely. Many small fragments are perfectly transparent, 

 and of a pale-lemon yellow or amber color ; the greater part of the ore 

 is, however, more or less opaque , and varies in color from Hght-yellow 

 or grey to brown or even dull black. Some of the more opaque mineral 

 is cellular in structure, shows no cleavage, has a dull rough surface, 

 and breaks with an uneven fracture. This may be an alteration pro- 

 duct of true stibiotantalite, one such specimen yielding an amount 

 of water of combination expressed by the formula (SbO)2 TajOe ^HjO. 

 Such a fragment had a specific gravity of only 4-77. In the unaltered 

 stibiotantalite Goyder found a range in specific gravity from 6-47 to 

 7-37. The author has made the following specific gravity determina- 

 tions, amongst others :— 6-41, dark-yellow pebble ; 6-75, grey pebble ; 

 7-18, grey pebble, with yellow veins, strong cleavage ; 7-43, pale-lemon 

 yellow, indistinct crystal ; 7-48, dark -yellow crystalhne ; 7-50, pale 

 greyish yellow, indistinct crystal. The bulk sample from which most 

 of these pieces were taken assayed tantahc oxide, 50-57 per cent. ; 

 niobic oxide, 12-58 per cent. A second bulk sample assayed tantalic 

 oxide, 51-13 per cent. ; niobic oxide, 7-56 per cent. 



The stibiotantalite occurs in fragments from the size of a pin's 

 head up to about lin. in diameter. A single exceptionally large crystal- 

 line mass, about 2|in. in diameter, is in the possession of the Foote 

 Mineral Company. 



This mineral has been examined for radio-activity, with negative 

 results, by Messrs. Mawson and Laby. 



CONCLUSION. 

 In conclusion I desire to express my indebtedness to several gentle- 

 men for the material information obtained from their letters or published 

 papers. Amongst others, I am specially indebted to Messrs. A. Gibb 

 Maitland, C. W. Marsh, G. A. Goyder, D. Mawson, and J. C. H. Mingaye. 



13.— THE FORMATION OF GLACIAL VALLEYS AND LAKES 

 IN SOUTHERN NEW ZEALAND. 



By PERCY MORGAN, New Zealand Geological Survey. 

 [Abstract.] 

 All New Zealand geologists agree that during late Tertiary or 

 recent time there was a great extension of the glaciers in the South 

 Island, but there is less agreement as to the influence of ice on the 

 topographical features of this region. The writer takes a rather 

 moderate view, and gives prominence to faulting as a factor in the 

 formation of glacial valleys and lakes. 



