Srymour—WHetamorphism of a Porphyritic Andesite. 9571 
with the rock at Ballymooney. Hence, while the latter is highly 
schistose, the rock sheltered by the granite ridge is comparatively 
speaking, unaltered. In this locality, therefore, the rock may be 
obtained in a practically unsheared state; but it is yet very far 
from being in its original condition, for it has been completely 
thermo-metamorphosed by the granite. In its unaltered condition 
the rock was undoubtedly identical with the “ Lambay por- 
phyry,”’ the well-known porphyritic andesite occurring on Lambay 
island and on the shore at Portrane. 
The thermo-metamorphism of an andesite of similar type has 
been described in detail by Messrs. Harker and Marr,! and the 
rocks near Donard have gone through similar stages of alteration. 
The final result has been a complete re-crystallization of the 
original constituents of the ground-mass which now consists chiefly — 
of a bronzy-brown mica, associated with a minutely crystalline 
mosaic of clear felspar and quartz only occasionally distinguishable 
from one another in thin sections under the microscope. Starting 
then with the Donard rock in this condition, its subsequent altera- 
tion by pressure may be readily traced in the field by the changes 
undergone by the porphyritic felspars, which stand out conspi- 
cuously on a weathered surface. Prior to any shearing effects 
these phenocrysts are seen to be arranged in an irregular manner 
(fig. 1, Pl. xxvr.), and form stumpy prisms of varying size up to a 
maximum of 1:5 x 1 x 0:5 cm. scattered singly or less often in 
glomero-porphyritic groups (fig. 1, Pl. xxvit.) throughout the rock. 
The crystals on a fresh fracture are rather brownish in colour owing 
to numerous secondary inclusions, but still show several parallel 
light reflecting bands due to repeated twinning. The first modifica- 
tion produced by pressure is to cause these plagioclases (andesine or 
labradorite) to arrange themselves in a direction with their longer 
axes approximately parallel to one another (fig. 2, Pl. xxvi.). They 
are in this condition not very much elongated, though their cross- 
sections have become somewhat flattened. The next stage con ists 
in the gradual drawing out of the crystals which become longer 
and longer (fig. 3, Pl. xxvit.), till finally they become leaf or torpedo- 
shaped, flattened lenticles up to 6 cm. or more long, about 1 cm. 
broad, and varying in thickness at various points from 0-1 to 1 mm. 
1 Quart. Journ. Geol. Soe., vol. xlvii., pp. 293-300. 
