494 J. Allan Thomson—Inclusions in Volcanic Rocks. 
A. EwnatztocEnous Inciustons. 
1. Cognate Xenoliths. 
Cognate xenoliths have already been noted by Professor Cole in 
dolerite veins cutting the olivine dolerite.’ They consist of ‘‘the 
compacter and basaltic layer’’ which the veins have entered. Those 
collected by me may be of this category, though I did not observe 
that the including rock was a vein. ‘They are of a fine dolerite in 
a coarse one. They are of a grey flinty appearance with frequent 
crystals of pyrites, and are irregularly mottled with dull greenish 
patches. 
In thin sections they are seen to consist of an aggregate of small 
felspars, generally in short stout prisms, but often rounded, with 
patches of irregularly-shaped augite grains, which extinguish 
simultaneously; it is an ophitic structure in which the optical 
continuity remains, but the actual continuity of the augite (in section, 
at least) is broken. It is these plates of augite which give the hand- 
specimens the mottled appearance. ‘The felspars run from labradorite 
to anorthite, and preponderate largely over the augite. Olivine is 
absent. The rock is thus a fine-grained ophitic dolerite. It has 
suffered no great change from its reheating. 
2. Accidental Xenoliths. 
The alteration of the Lias at its contact by the intrusive dolerite 
(here basaltic) has been fully described by Professor Cole in the paper 
already referred to. But near the shore contact it is not uncommon 
to find patches of the Lias included in the basalt, and where this is 
the case the contact phenomena are much more marked. Professor 
Lacroix has observed? that along the shore the Lias has been changed 
to a rock formed mainly of cordierite and extremely rich in crystallites 
of pyroxene and a little magnetite, with here and there small grains 
of quartz intact. This alteration is somewhat different to that 
described by Professor Cole, who has found no cordierite. According 
to my specimens, the Lias some little distance from the contact 
contains a good deal of quartz and magnetite and some pyrite in 
elongated patches, around which is recrystallised quartz, a few 
scattered flakes of biotite, some chlorite, numerous small granules 
with a high index which seem to be pyroxene, and a few colourless 
crystals which may be cordierite; but the fineness of grain and the 
overlapping of the individual crystals makes identification of all the 
constituents difficult. A few inches from the actual contact the 
rock is still fine-grained, but the granules of pyroxene are more 
recognisable. The rock also contains patches of colourless minerals 
of larger size, most of which are felspars with Carlsbad twins, but 
occasionally albite twinning also occurs and anorthite can be 
recognised. Round the borders of the patches the pyroxene granules 
are more closely studded and of slightly larger size. Rarely a 
pleochroic mineral occurs in the patches. From its very great relief 
and the strength of the birefringence it agrees with staurolite rather 
1 Loe. cit., p. 63. 
2 «¢ Tes Enclaves des Roches voleaniques,”’ pe 6556 
