J. Allan Thomson—Inclusions in Volcanic Rocks. 495 
than with andalusite, which Professor Cole has found in the con- 
tact zone. 
It is only among the actual inclusions that I have found specimens 
similar to those described by Professor Lacroix. The presence of 
cordierite in minute crystals is difficult to verify microscopically unless 
basal sections are found giving the characteristic twins. In none of 
my sections can these be found, but they resemble so much Professor 
Lacroix’s sections, which he kindly lent me for examination and in 
which he was able to obtain this verification, that I do not hesitate to 
identify as such a mineral occurring abundantly in roughly quadrate 
plates with straight extinction compared to the direction of elongation. 
It has in general a lower interference colour than the quartz which is 
also present, and it is crowded with inclusions of magnetite and fine 
granules of pyroxene. 
Another type of inclusion is quite different from any rock hitherto 
described from this locality. The hand-specimens differ from the 
altered Lias only in the presence of streaks of a dull greenish-white 
colour. In thin sections the darker part is seen to consist of granules 
of a green pyroxene, much larger than those in the contact specimens, 
interspaced by colourless prisms and grains of a felspar which 
occasionally includes the pyroxene. The lighter-coloured streaks are 
formed, some of felspar with subsidiary wollastonite, some of felspar 
lime-garnet and calcite, with scattered grains of the green pyroxene. 
This latter mineral shows an occasional cleavage trace and is faintly 
pleochroic. Extinction angles up to 42° (CAC) on longitudinal 
cleavages have been observed. The pleochroism in thick sections is: 
a=pale grass green, P=yellow green, C=blue green. The felspar is 
most easily determined in the colourless patches, where it is of largest 
size. The extinction angles with reference to the trace of the plane 
of symmetry of the albite twins show it to be anorthite. The lime 
garnet is confined to a few of the streaks. It occurs in irregularly 
rounded grains. The wollastonite has a habit not unlike anorthite, 
from which it may be distinguished by its slightly higher refringence, 
and by the variable sign of the elongation of its sections; those giving 
positive elongation, and under convergent light an optic axial cross 
perpendicular to the elongation, may with certainty be referred to 
wollastonite. It does not occurin great quantity. The rock therefore 
consists largely of lime silicates and was probably a calcareous band 
in the Lias. 
The study of these inclusions is thus seen to give valuable aid in 
the study of the contact phenomena of this rock. Professor Cole 
suspected wollastonite,' but was unable to verify its presence, nor 
were the colourless granules of sufficient dimensions to furnish him 
means of determining the felspar. The development of cordierite in 
the argillaceous inclusions of basaltoid rocks is mentioned by Lacroix 
as very frequent.* In the case of the Portrush rock he considers it 
‘< little doubtful that a part of the elements necessary to its formation 
are of exogenous origin.” * Limestone inclusions are generally simply 
1 Loe. cit., p. 59. 
2 Loe. cit., p. 49. 
3 <« H’tude sur le Métamorphisme de Contaet des Roches Voleaniques,”’ p. 47. 
