84 H. J. JOHNSTON-LAVIS ON THE GEOLOGY 
of sanidine crystals besides a few of amphibole and garnet. To the 
unaided eye it is undistinguishable from the equivalent in Pass VI. 
Period 4. 
Microscopical examination.—The sanidine is of the usual kind. 
The amphibole exists as small crystals and large microliths of the 
ordinary dark green tint. There are some large well-formed plates 
of biotite and a good many beautiful hexagonal sepia-coloured 
microlithic plates of the same mineral, remarkable for their perfec- 
tion and purity. 
Melanite, rather light in colour, occurs in well-formed crystals. 
In one case there was a melanite crystal traversed by one of amphi- 
bole, the prism of which, however, thinned as it approached from 
either side towards the centre of the garnet, showing that these 
two minerals were contemporaneous in their growth, although the 
amphibole had the start. There are a few crystals of pyroxene ana 
also a few microliths. 
The most obvious fact that distinguishes this pumice from all we 
have yet studied is the enormous number of microliths of leucite. 
These are seen usually as transparent polyhedral sections whose 
outlines are well marked by the layer of dusty glass that surrounds 
them. This dusty matter seems from time to time to have increased 
to such an extent as to impede the regular growth of the crystal, 
which was compelled as it enlarged to envelope strata of this dusty 
matter so as to form glass-cavities parallel to the crystalline facets. 
A few rod-like microliths are often entirely enclosed, or may project 
from the surface of an individual leucite crystal. The crystals or, 
more properly, microliths of this mineral are so small that they 
remain dark between crossed nicols, though a few of the largest ex- 
amples may, by care, be made to transmit a very feeble grey light, 
but not strong enough to exhibit the peculiar and characteristic 
striation of larger crystals of leucite. 
The matrix is a clear and colourless glass, very vesicular; the 
cavities are not uncommonly drawn out into long tubes so as to give 
a fibrous appearance to the rock. 
General Description.—The middle subdivision, or upper of the 
pumice, is the rather gradual sequent of that we have just described. 
Towards the upper part, where its special characters are fully de- 
veloped, the pumice is a very compact brownish or, rarely, greenish 
erey-coloured rock. It usually encloses many erratic fragments 
and crystals. 
Microscopic Structure.—All the elements that existed in the white 
pumice, are to be found in this, except that most of the minerals 
are larger and more extensively developed. The principal difference 
is in the number of pyroxene crystals and microliths. 
The darker colour of the rock is due to the much greater deve- 
lopment of the dirty material surrounding the leucite, which seems, 
under the highest powers, to be composed of minute rods, probably 
pyroxene, and minute globulites, which may be magnetite partially 
altered, so giving a brownish tint. 
There still remains a considerable quantity of interstitial glass, as 
