of Edinhurgh, Session 1883-84. 
483 
Polysyntlietic individuals, made up of repeated twins on the albite 
plan, were very rarely observed. The felspar in its optical pro- 
perties is thus seen to lie between lahradorite and hytownite. 
The twin growths are particularly frequent and interesting on 
account of the structure of the individuals. In addition to those 
of the albite type, others were observed in which the edges P/M 
and P/K could be definitely determined as the axes of twinning, 
whilst P and K formed the twinning planes. The plane of composi- 
tion was principally either P or M when penetration twins were 
not observed. 
These fragments and crystals of plagioclase contain inclusions of 
vitreous matter, and sometimes grains of magnetite. Perhaps a 
small number of felspathic grains may belong to sanidine, the 
presence of which Is insinuated by the percentage of potass indi- 
cated by the analysis which follow (K 2 O — 0*97 per cent.). 
We have said that the pyroxenic minerals of the ash are augite and 
a rhombic pyroxene ; we distinguish them by the microscope some- 
times in the form of fragments— and this is usually the case— some- 
times in the form of crystals, which we can isolate from the 
volcanic glass covering them by treating them with hydrofluoric 
acid. In the crystals of augite we distinguish the faces of a 
prism, of the brachypinakoid, and indications of the faces of a 
pyramid. This augite is pleochroic and has a greenish tint, and 
extinguishes in certain cases obliquely to the prismatic edges. It 
is this character which often permits it to be distinguished from 
rhombic pyroxene with which the augite is associated. The 
crystals of hypersthene are transparent, of a deep brown colour, 
strongly dichroic, with green and brown tints. They are in rect- 
angular prisms terminated by a pyramid, and extinguish between 
crossed nicols parallel to their longitudinal edges. Magnetic iron, 
which is rather abundant in the ashes, is recognised in the form of 
grains and octahedrons. We have not been able to detect with 
certainty either hornblende or olivine. The largest grains of this 
ash are true microscopic lapilli, where we distinguish in a vitreous 
mass microlithic crystals of felspar, of magnetite, and more rarely 
of pyroxene, Finally, we observe with the microscope particles of 
an organic origin, which are easily recognisable by their fibrous and 
reticulated structure. These impurities may have been transported 
2 I 
VOL. XII. 
