76 H. J. JOHNSTON-LAVIS ON THE GEOLOGY 
The pyroxene crystals are few and far between, and when they 
do occur may have a nucleus composed of a fragment of amphibole. 
Black mica or biotite is rare. The mineral most common after 
amphibole is melanite. It is scattered rather commonly throughout 
the matrix as well-formed crystals about the size of a turnip seed or 
larger. When in very thin sections it transmits a rich reddish-brown 
light, and contains various enclosures, chiefly minute transparent 
microliths. This mineral and sanidine, from the fracture of the one 
and the cleavage of the other, and especially from the hardness of 
the former compared with the spongy and fragile matrix, render the 
sectionizing of this rock a very precarious proceeding. Even with 
all the precautions conceivable these minerals tear out before the 
rock is sufficiently thin, so that it is advisable to prepare thick sec- 
tions for the study of these minerals, and thin ones for the base or 
glass, microlithic or not, as the case may be. 
Magnetite occurs as well-formed octahedra, but is very rare. 
Scattered throughout the base are a number of microliths, which 
seem to consist principally of pyroxene, though they are quite small 
in quantity in proportion to the amorphous base. 
The ground-mass proper is a clear almost colourless glass, filled 
with vesicular cavities, which assume their usual varied forms, and 
sweep round crystals or larger cells, or are drawn out into long 
but slender tubules, showing the great viscosity of the igneous 
matter. 
The pumice of the middle subdivision is compact, sinks in water, 
and is usually grey in colour with a distinct greenish tinge, due to 
the abundance of pyroxene microliths ; when it 1s weathered it may 
become brown from the peroxidation and hydration of the magne- 
tite. When broken open there are to be seen many irregular 
cavities, usually containing pieces of altered limestone, which have 
a coating of white crumbling powder, which is probably hydrate of 
lime. Besides these enclosures there are other impurities, ranging 
from the finest fragments of crystals to pieces of old lavas, and meta- 
morphic erratic blocks of large size. There are amongst these 
enclosures minerals that are of uncertain origin, such as large crystals 
of biotite, pyroxene, melanite, olivine, idocrase, leucite, &c., which 
may be perfect or broken and eroded. Pyroxene occurs either in 
the pumice or as loose crystals mixed with the rock fragments, in 
size fsometimes as much as one or two centimetres in diameter, 
usually broken and of a dark green colour, with poorly developed 
cleavage, so that the fracture appears vitreous. Melanite may be 
well formed when small, or irregular and broken when large. The 
same remark applies to olivine and leucite, both of which are rare 
and difficult to find, especially the latter. All these have the stamp 
and appearance of minerals derived from extraneous sources and 
accidentally enveloped in the plastic magma. They all resemble 
very strongly both in form and in their minute characters the same 
species that are met with in the ejected blocks of metamorphic rocks, 
from which they may be supposed to have been separated by the 
violence of the explosion, and then enveloped in the pumice paste. 
