200 
also as a thin outer zone surrounding the augite and parallel 
with it. 
Biotite of a brown colour is found in rather considerable 
amount. It occurs as aggregates of small scales or plates 
round the iron-ore, sometimes also in large, tabular crystals 
surrounded by felspar. It is strongly pleochroic, the absorp- 
tion-tints varying from deep chocolate colour to quite pale 
yellow. 
A labradorite in rather idiomorphic, tabular crystals is the 
main component of the essexite. The crystals are often several 
centimeters long. They are always twinned according to the 
albite-law. locally also pericline-lamellæ are seen and Carlsbad- 
twins are tolerably common. Upon optical examination the 
main part of each crystal proves tolerably homogeneous but in 
an outer shell of varying thickness a decrease of basicity takes 
place. This zonary banding is as a rule very pronounced. For 
a more exact determination of the plagioclase, measurement 
was employed of the angles of extinction in symmetrical sec- 
tions of Carlsbad-twins and in sections parallel to 5 (010). The 
main portion of each crystal was in this way found to consist 
of a labradorite the composition of which varied in the different 
specimens of the rock from Ab,An, to Ab,An,. In the outer 
part of the crystals the extinction-angles decrease and attain 
values characterizing an andesine Ab,An,, and sometimes the 
oulermost zone is oligoclase. 
In some specimens of the essexite the plagioclase is quite 
clear and fresh, in others it is filled with minute dust-like 
inclusions. When the felspar contains small enclosed biotite 
crystals the dust is often wanting in the immediate vicinity of 
these’. In many cases the plagioclase is more or less altered 
and the crystals are filled with small scales of a colourless 
1 A similar micro-structure, of the labradorite has also been observed in 
essexiles from other localities (Возехвозсн, Mikrosk. Physiogr. 1907, II, 
lp 392). 
