NOTES ON A ROCK FROM NEW ZEALAND. 
209 
The rhombic pyroxene also occurs only sparingly. It is 
distinguished at once from the hornblende by its extinguishing 
when the traces of the prismatic cleavage in approximately 
longitudinal section are parallel to the axes of the Nicols,by the 
nearly right-angled prism which is shown by the cleavage in 
cross sections, and by the different dicliroism, the colour of 
the vertical sections varying from a reddish brown to green 
during a rotation when the polarising prism only is used. 
The felspar is both orthoclase and plagioclase, but I 
have not been able to determine the exact character of the 
latter. The chief point of interest is that the crystals are 
frequently mere skeletons of felspar substance enclosing a 
perfect swarm of glass inclusions. These are of most 
irregular shapes, sometimes very large at other times of but 
small size. They almost always appear slightly brownish in 
colour and with but few exceptions are provided with a 
spherical cavity. 
Seeing that these cavities obviously vary a good deal in the 
proportion they bear to the size of the inclusions which contain 
them, it seems reasonable to conclude that they are at any 
rate in part air bubbles, and it is quite possible that their 
presence may have determined the formation of the inclusions. 
As is so usual, it is frequently noticed that the outer zone of 
one of a felspar crystal is much more clear and continuous 
than the central parts, as if the crystallisation went on more 
quietly and regularly towards the end of the cooling process. 
A few instances of strain polarisation may be observed in 
the glassy base, and occasionally cracks closely surround 
crystals of the rhombic pyroxene, but the phenomenon is not 
very striking. 
As all present will no doubt remember the terrific explosive 
eruption of Mount Tarawera, last June, I need not spend 
much time on a description of the damage done by it. 
It may be interesting, however, to note that the usual 
linear character of the volcanic action was well shown. If a 
map of the volcanic region of the North Island is consulted 
and a line drawn from Mount Tongariro, in the S.W., to White 
Island, in the Bay of Plenty—both of them active volcanoes 
in a small measure—there will be found lying on it both 
Mount Tarawera and Lake Rotomaliana, into which the lovely 
white and pink terraces, the result of the deposition of silica 
from boiling springs on the hillside, descended. 
These, alas, have been utterly destroyed. The hot springs 
were evidences of a weak place, and when the explosion 
occurred it was among those which gave way. 
