68 
border and the central mass of essexite a transitional zone 
consisting of a rock which is dark in colour and thus 
resembles the essexite, but which is characterized by the 
presence of large porphyritic feldspars, sometimes as 
much as two inches in length, of peculiar form scattered 
through it and often arranged with their longer axes in 
the same direction, thus giving a fluidal appearance to 
the rock. This rock contains a large proportion of the 
same iron-magnesia minerals, more especially the horn- 
blende, found in the essexite, and passes over gradually 
into this rock. Its passage into the pulaskite is rather 
more abrupt and is marked chiefly by the almost entire 
disappearance of the dark-coloured constituents above 
mentioned. There is, however, a continuous transition 
from the pulaskite through this intermediate rock into 
the inner essexite of the mountain. 
This transitional rock is composed of the same miner- 
als as the essexite with the exception of the feldspar, 
which consists in part of the soda-orthoclase characteristic 
of the pulaskite, and in part of the plagioclase (in this 
case oligoclase) which forms the feldspathic element of the 
essexite. It is thus in mineralogical composition inter- 
mediate between these two rocks, although, as above 
mentioned, being rich in the dark coloured constituents, 
it more closely resembles the latter. 
The large feldspars have frequently a peculiar crys- 
talline form giving to the mineral, when broken across, a 
perfect hexagonal outline. The six faces represented in 
this form are apparently T, L, and M. The crystals 
hold many little inclusions of pyroxene, biotite, hornblende, 
magnetite, sphene and nepheline, often regularly arranged 
so as to give a zonal structure to the feldspar individual. 
The specific gravity of twelve small fragments of the 
feldspar of these large crystals, collected from a locality 
on the southern side of the mountain, and as free as possible 
from all inclusions, was determined. The specific gravity 
of nine of these lay between 2-59 and 2-607, while that 
of the other three was between 2-625 and 2-628. This 
shows the feldspar in the former case to be identical with 
that of the pulaskite, while in the latter three the specific 
gravity lies between that of albite and oligoclase. The 
somewhat greater specific gravity in this case may be due 
in part to inclusions of other minerals. A separation 
of the constituents of the rock shows, however, that, as 
