94 H. J. JOHNSTON-LAVIS ON THE GEOLOGY 
centimetres across, was perfect, being free from fracture or inter- 
penetration of other individuals, and is one of the finest specimens 
yielded by this volcano. The leucite is rarely pure, and often 
encloses crystals of amphibole or pyroxene; others partly or entirely 
envelop large crystals of sanidine, showing that here the leucite was 
formed later than the felspar. One may shell of from some indi- 
viduals a coat of variable thickness, leaving an exact model on a 
smaller scale in the nucleus, the line of separation being generally 
a stratum of glass-cavities or microliths, the whole reminding one of 
the so-called capped quartz. So abundant are these zones of impu- 
rities that the crystals break with great ease along these planes, 
giving a pseudo-cleavage parallel to the crystalline facets. The 
most interesting facts to the geologist are the evidence this mineral 
affords of the great churning and oscillation of temperature that the 
viscous magma must have undergone. As already mentioned, most 
oi the crystals are broken, others show partial repair, whilst others 
again have been in part re-fused, so that their angles and edges are 
all rounded. We may also find crystals fractured and the pieces 
only slightly displaced from one another; in other cases the crystal 
is crushed and flattened out as the result of compression in an almost 
solidified condition of the matrix. The most imteresting examples 
are crystals in which all the crystalline edges and angles have been 
removed so as to reduce the crystal to a sphere or agglomeration of 
spheres. 
; Large crystals of sanidine and pyroxene, and plates of biotite may 
be easily seen by the naked eye, besides a few grains of olivine. 
Microscopical Examination —tThe leucite shows to perfection its 
special optical properties between crossed nicols. Besides the enclo- 
sures of other minerals, there are a vast number of glass-cayities, 
some of which are very remarkable. One of fusiform shape had 
projecting into it from its extremities two microliths, placed m the 
same line with each other, but their two extremities separated a 
certain distance. Near the end of each of these rods was a vacuole 
in the clear ight sepia-coloured glass. 
The pyroxene is usually very well formed, though it may be much 
broken. It is usually of a light-green colour, though there is every 
shade between white and very dark green, and we may see Many 
crystals enveloped in a darker coat than their interior. The resem- 
blance of the darker specimens of this mineral to amphibole is such 
that in many cases it is almost impossible to distinguish them. The 
peculiar variation in colour of the pyroxene is not an uncommon 
thing, and is well seen in the same mineral from other localities, such, 
for instance, as the beautiful prisms from Tyrol. 
The biotite has the usual colour and character. The plates are 
often torn asunder, or twisted in various manners. Where a crystal 
has its plane of cleavage perpendicular to the plane of the section 
the plates may be seen to have been ruffled and frayed up, and 
pleated and folded back, showing the kneading action to which the 
semiviscid or plastic mass must have bees subjected. Melanite and 
magnetite both occur im large and pretty well formed crystals. 
