550 JOURNAL OF SCIENCE. 
both groups, but particularly in that of the plagioclase rocks, both 
sections are connected so intimately, that they cannot be distinguished 
with certainty, either macro- or microscopically, nor indeed generally 
without the determination of the per centage of silica. I was 
unfortunately limited to that, as I had no time for complete analyses. 
I found of silica— 
For liparites ” abs £ 69.99. to 78.37 7 
», trachytes -.... axe Bs 62.02 ,, 63.53 ,, 
», andesites ... ee 5 66.35 ,, 66.50 ,, 
5, basalts nit See A | 47.06 ,, 47.77 ,, 
With the aid of these determinations the liparites and trachytes could 
be fixed satisfactorily, but not the andesites and basalts, so that I 
prefer to describe them together, divided into two parts according to 
their geognostic occurrence. 
I. LIpaRiteEs. 
The liparites coming from Gebbie’s Pass, Gebbie’s Knob, Lyttelton 
Harbour caldera and Quail Island are light-grey or white coloured 
rocks. Of those varieties rich in silica, one (with {78.37 %/) is thinly 
laminated and is composed of white, light-grey, and light-yellow 
layers which are at the most 1 mm. thick, bent in a wave-like manner, 
and having their planes of separation rough and wrinkled. The 
hand specimen greatly resembles the slaty quartz-porphyries which 
are found at the Wagenberge near Weinheim in the Odenwald and 
described by Cohen. As in the latter, so in the former the few 
interpositions of quartz and feldspar extend through several layers, 
or the latter bend in a wave-like manner round them so that the 
interpositions must have existed when the lamination took place. 
A second variety is massive throughout and rich in interpo- 
sitions of dark smoky-grey quartz and white feldspar with glistening 
cleavage planes. In the quartz which is mostly well formed the prism 
sometimes appears quite subordinated. 
The varieties with less silica (70 to 72.75 %) are nearly free from 
macroscopic interpositions and have a clay-stone-like appearance, 
which—as well as its slight hardness—must probably be ascribed to: 
a commencing alteration. One piece from Gebbie’s Knob shows 
columnar separation. An occasional brown streak is due to the 
presence of secondary hydrated iron oxide. Under the microscope 
the ground-mass appears to be very variously formed; in one case 
the structure is microcrystalline, in another cryptocrystalline, in 
another granophyric, but though the one kind generally prevails 
strongly, it never appears quite alone. The laminated variety shows 
a nearly uniform granophyric structure. Apart from single large 
quartz grains and small portions of micro- to crypto-crystalline 
ageregates, the ground-mass consists of fibres which are arranged in 
concentric bundles. Since these bundles now and then change by 
‘degrees into micropegmatic intergrowths in which quartz and feld- 
spar are to be recognized side by side, we may suppose that the fine 
fibres also consist of these two minerals. At any rate among the 
fibres there are some which belong to a monoclinic mineral, for in 
the interference cross which frequently appears, the arms of the cross 
form with the principal sections of the Nicols, angles up to 14 degrees. 
Under high powers the fibres of the granophyric bundles appear 
vermiform. 
