IIARKER : PETROLOGICAL NOTES. 
417 
prisms up to one-tenth of an inch in length, patches of dark mica 
to a quarter of an inch, and confused dark spots. 
Micro. [1056]. The porphyritic felspars, when sufficiently clear, 
show fine twin-lamellation and extinction-angles proper to labradorite. 
There are roughly rectangular sections of a fibrous, green, pleochroic 
substance, with indistinct polarisation and straight extinction, doubt- 
less bastite pseudomorphs after hyper sthene. Some paler green and 
more amorphous patches are derived from augite, of which a few 
unaltered granules have sometimes survived. This mineral was 
subordinate in quantity to the rhombic pyroxene. Biotite flakes, 
also considerably altered, are occasionally seen, and little grains of 
magnetite are fairly abundant. The ground-mass seems to have 
consisted . largely of felspars, but is deeply decomposed, and stained 
reddish-brown by feriic oxide, which has also coloured the borders 
of some of the porphyritic felspar-crystals. H5^persthene-bearing 
rocks similar to this occur in place in the Cheviots. 
The next two specimens, Nos. 174 and 202, also belong to the 
type usually described as porphyrites. They show a dull compact- 
looking ground of a dark purplish-brown colour, enclosing glassy 
felspars one-tenth to one-quarter of an inch long, or sometimes 
larger, and little brown pseudomorphs. The rocks are evidently 
andesitic lavas. 
Micro. [1147, 1148]. The large felspars, which are quite fresh, 
show lamellar twinning on the albite law, often interrupted and in 
many cases terminating abruptly at a crack in the crystal. There 
are well-marked zones of growth, besides a more irregular patchy 
appearance. The ground-mass is composed mainly of small closely- 
lamellated felspars, which have an elongated form and the parallel 
arrangement due to flow. This gi'ound, however, is packed with 
opaque matter, part of which shows forms indicating octahedra of 
magnetite, while the bulk of it is red-brown by reflected light and 
consists of ferric oxide. One or two rounded crystals of pale augite 
are seen, and some pale-green decomposition-product may also 
represent a pyroxenic mineral. Calcite is another secondary 
product, and often occupies patches which seem to be vesicles. 
The pseudomorphs already referred to are surrounded by a black 
