J. Allan Thomson—Inclusions in Volcanic Rocks. 497 
olivine. The augite is fresher and is a pale variety, but is in part 
replaced by chlorite. A little brown hornblende is present. Calcite is 
abundant and apparently fills vesicles. The groundmass in both cases 
is very dark with ferruginous decomposition products. In the first 
type, a little altered plagioclase felspar is present; the groundmass is 
more glassy and contains small granules of augite and olivine. The 
rock has been an olivine basalt. In the other the felspar is absent 
and the augite.in the groundmass is elongated into prisms. The rock 
appears to have beenalimburgite. The character of the homceogenous 
inclusions strengthens the supposition that the magma has been an 
ultra-basic one. All the other basalts recorded in the district contain 
felspars, and in none are the inclusions similar. 
The hornblende is generally without any corrosion border, but 
occasionally well-rounded crystals are found including large crystals of 
magnetite. It is probably to this corrosion that the polished and 
rounded surfaces of the hornblende crystals in the tuffs are due. In 
one slide a large crystal of augite also showed corrosion phenomena. 
Homa@ogenous INcLusIons. 
1. Lsolated Crystals. 
Numerous minerals occur in isolated fragments in the tuffs and 
breccia, and sometimes attain a tremendous size, especially felspar, 
hornblende, and a black augite. Garnets, a green augite, biotite, and 
olivine occur in smaller fragments. The hornblende, garnet, and 
black augite are sometimes beautifully polished and rounded. With 
the exception of the felspar these minerals can all be traced to the 
inclusions described below. Its origin has remained an enigma. It 
occurs in all sizes up to 70 mm., elongated in the direction of the two 
principal cleavage faces. Crystal faces are wanting. It is transparent 
and colourless except where the cleavage planes are coated with 
calcite, in thin sections perfectly clear and free from inclusions. It is 
twinned according to both the albite and pericline laws, and the 
extinction angles and a partial analysis show it to be an acid 
oligoclase. 
2. Holocrystalline Rocks. 
These occur in great abundance. The commonest type, in fragments 
with diameters up to eight inches, is an olivine nodule containing 
much calcite. The more compact types are smaller, and contain 
olivine only in subsidiary amount. A totally distinct type consists 
primarily of a black augite and garnet. All these rocks are holo- 
erystalline and granular with fairly coarse texture. None of the 
elements have crystalline forms, but the spinels and the hornblende 
which they contain are posterior to the other minerals. They may be 
conveniently divided into three groups, these being characterised by 
the presence of spinel and a green augite (diopside or diallage) in the 
first, a black augite in the second, and predominant basaltic horn- 
blende in the third. The following table will show the different 
mineral combinations :— 
DECADE V.—VOL. IV.—NO. XI. 32 
