48 
J. Y. ELSDEX—MICROSCOPIC STRUCTURE 
Under the microscope the felspar * is seen to be wholly triclinic, 
occurring both in long, prismatic crystals, exhibiting the usual 
coloured striae in polarised light, and in crypto-crystalline masses. 
It frequently contains long needles of apatite , which are especially 
abundant in the crypto-crystalline portions. With the exception 
of a little turbid grey matter, and a few brownish-yellow streaks 
in some of the crystals, the felspar appears but little altered. 
Augite occurs in well-formed brown and reddish-brown crystals, 
and in irregular grains : some of the forms exhibit the character¬ 
istic angle of augite, and have also a feeble dichroism, changing 
from purple-brown to yellowish-brown. Twin crystals appear to 
be rare. Magnetite is frequently enclosed. Olivine is plentiful, 
and has the usual appearance of alteration along lines of fracture : 
in some cases the whole crystal is changed to a yellowish-green 
chloritic or serpentinous substance, the margins being marked by 
a dark band of magnetite. Magnetite is present, both in large 
black patches, and as minute crystals enclosed in the other minerals. 
A few fragments of biotite may also be noticed. 
Ashwell, 123 (PI. Ill, f. 2).—A tolerably fresh, dark, crystalline 
rock, with green grains of olivine plainly visible. 
Under the microscope the felspar appears to be wholly plagio- 
clase, and but little altered. It contains numerous needles of 
apatite. In some portions of the rock, however, the whole of the 
felspar appears to have been converted into chloritic pseudomorphs. 
This is a marked feature of the rock, and should be useful for 
identification. Augite occurs both in isolated crystals and in 
aggregates. It polarises brilliantly, is but little decomposed, and 
contains crystals of magnetite. A little biotite appears, also, to be. 
present. Olivine is present in large crystals, somewhat altered into 
a brownish-yellow substance {chlorite'), and contains magnetite, occa¬ 
sionally altered into bright red hematite. Both magnetite and titan - 
iferous iron occur scattered through the rock, and there are several 
red stains of ferric oxide, produced by secondary decomposition. 
Ashwell, 90. —A dark crystalline rock, apparently fresh, and 
showing distinct crystals of felspar, pyroxene, and olivine. 
Under the microscope the felspar is seen to be triclinic. It is 
abundant, and fairly unaltered, although occasionally converted 
into the usual grey granular substance. Augite occurs in isolated 
crystals, and in groups, and polarises brilliantly. Olivine is not so 
abundant as in the preceding specimens. It is sometimes com¬ 
pletely altered into a chloritic substance, and often shows the 
characteristic mode of alteration in lines perpendicular to the 
fissures. It frequently contains crystals and grains of magnetite. 
Magnetite is plentiful, often in square and triangular sections. 
Some apparently rhombohedral forms of titaniferous iron may 
be noticed. Apatite is fairly abundant. A little biotite may be 
noticed. A few plates of a brownish-green chloritic substance, 
similar to the olivine pseudomorphs, occur in places. 
* The names of the minerals detected by the aid of the microscope are printed 
in italics. 
