100 H. J. JOHNSTON-LAVIS ON THE GEOLOGY 
Let us now direct our attention to the primary material and its 
Microscopical Structwre.—The most remarkable are the leucite 
crystals, which are moderately pure, though they do contain amphi- 
bole and pyroxene, besides glass-cayities. 
Next are a large number of crystals that might at first be taken 
for amphibole from their dark colour and considerable polychroism ; 
but the measurements of the angles of prism-sections show them to 
be pyroxene. ‘There are others that are undoubtedly amphibole; but 
there are many that, from want of sufficient evidence, remain unde- 
termined. ‘The colour is very variable, ranging from white to dark 
green, or the crystals may have a nucleus different from the exterior. 
The amount of cleavage is also most variable. Besides these doubtful 
ones are other large crystals of lighter colour, covered with a darker 
coat, about which there can exist no suspicion. The size ofall these 
is most variable, ranging from large macroscropic crystals down to 
minute microliths. The inclusions are chiefly magnetite and glass- 
cavities. One peculiarity about the larger crystals is the extremely 
flecky appearance that they possess, the cause of which is not very 
evident. ‘This irregularity may be confined to the central portion of 
the crystal, whilst the external part is clear, and shows fine striations 
or lines of accretion parallel to the crystalline faces. The light 
transmitted by the prisms of pyroxene at right angles to the principal 
axis 1s a pure green ; whereas, seen in transverse section, the hght 
being parallel to the principal axis, it has a brown tint. 
Biotite occursin well-formed and characteristic crystals of large size. 
Sanidine is abundant in large, well-formed, or broken crystals. 
They are remarkable for their dirty state, and showing a minute 
parallel striation. Other examples present an irregularly flecked in- 
terior, enclosed in a laminated and clearer coat or shell, similar to 
those in the pumice of Puasr VI., Period 2. The dirty appearance 
is chiefly due to a number of elass-inclusions. The mineral may also 
envelop pyroxene or sarinlbls. 
There are large, perfect crystals, hexagonal in outline, oie 
of magnetite or titaniferous iron. 
There are a few crystals that seem like idocrase, but, from their 
want of certain characters, and the fact that this mineral is usually 
of metamorphic origin, it is wise to examine more specimens before 
coming to any decision. 
The microliths are of leucite, pyroxene, biotite, sanidine, and 
magnetite. The last is seen in minute microcrystals of hexagonal 
and quadrangular outlines of a blood-red colour, showing the true 
nature of the mineral that has already been discussed as somewhat 
problematical. 
There remains usually a small quantity of dusty glassy base. The 
rock is, as a whole, very slightly porous, and in some cases remark- 
ably compact. 
Ftemarks.—This pumice is unique among all those we have studied 
for the remarkable development and perfection of its crystalline 
structure. In size, each component mineral species is gigantic 
compared with what it usually is in other cases. 
