HARKER : PETROLOGICAL NoTES. 
303 
Micro. The slice i.s seen to be deeply decomposed, but the 
large felspars still show a very fine cross-laniellation with narrow 
extinction-angles pointing to andesine. The slide contains plenty of 
a green chloritoid product, probably occup)'ing the place of pyroxenic 
minerals now destroyed. Large patches of secondary calcite also 
occur, and the only original minerals surviving in a fresh condition 
are apatite, in abundant hexagonal prisms, and irregular ragged 
grains of magnetite. The gTOund-mass consists mainly of felted 
microlites of felspar, but probably there has been a glassy residuum 
also. Portions of this ground-mass have been caught up by the 
porphyritic felspars. The rock is an altered andesite (' porphyrite ' 
of many authors), and presents a very close resemblance to those 
from the Cheviots. 
[935] From the Basement Clay at Bridlington Quay. A hand- 
some crystalline rock, shewing grey felspars and dark mica. 
Micro. The slice consists mainly of large felspar crystals. 
Some seem to be ordinary orthoclase ; others have the minute 
" cross-hatching" of microcline, and include intergrowths of finely- 
lamellated albite (?) : others again are of striated acid plagioclase 
containing numerous little rectangular patches of orthoclase with 
parallel growth. A curious feature is seen at the contact of the 
several crystal plates, where they seem to interlock in an intricate 
fashion recalling a micropegmatite intergrowth. There is plenty of 
pyroxene, pale-brown in section, of earlier consolidation than the 
felspar but enclosing magnetite grains. In addition to the augite- 
cleavage, it possesses a diallagic structure, which is often developed 
in the interior of a crystal but not at the margin. Good hexagonal 
prisms of apatite are very plentiful. The brown mica (biotite) is 
less abundant than appears from the hand-specimen : it penetrates and 
is enclosed by the felspar. 
[936] From the Basement Clay at Dimlington. A beautiful 
crystalline rock, resembling the last though darker in general appear- 
ance. The felspar shows on many of its crystals an iridescence like 
that of the well-known Labrador mineral. A black pyroxenic 
mineral and flakes of dark lustrous mica are also to be seen on the 
specimen. 
