304 
MARKER : PETROLOGICAL NOTES. 
Micro. The felspar appears to be mainly ortlioclase or soda- 
orthoclase, each crystal plate presenting, as a rule, an optically 
uniform field, though there is some veining with a different felspar- 
substance. The microcline structure is not detected. The pyroxene, 
grass-green in section, is for the most part typical diallage. As 
before, the " schiller" structure sometimes affects only the interior 
of a crystal, but, on the other hand, there is sometimes schilleriza- 
tion on iwo intersecting sets of planes. A system of parallel lamellar 
inclusions is often developed, though in a much sligliter degree, in 
the felspar, and may account for the play of colour noticed in the 
hand-specimen. The diallage is earlier than the felspar, but includes 
apatite in large and small prisms and magnetite in octaliedra and 
more complex shapes. Brown dichroic biotite is associated with the 
diallage, and also some hromatite. A few little crystals with very 
high refractive index and apparent tetragonal habit may be zircon. 
These two specimens represent a very peculiar petrological type, 
which 1 am not able to parallel except by the augite-syenites, so 
largely developed in southern Norway, as described by Brogger and 
others. Indeed, there can be practically no doubt that these boulders 
had their origin in that country. 
[937] From the Basement Clay at Bridlington Quay. A gneiss 
exhibiting light felspathic streaks or bands, half an inch or more in 
width, alternating with dark micaceous layers. There is also plenty 
of white mica in glistening flakes set parallel to tlie gneissic banding. 
Micro. The chief constituents are felspar, often in broken 
crystals, lenticular streaks composed of aggregates of clear quartz- 
grains, and bands of closely-packed mica-flakes, both muscovite and 
biotite. The felspar seems to be partly orthoclase, but is mainly an 
acid plagioclase with fine striation and often cross-lamellation. The 
crystals are often bent, and the twinning seems to be related to the 
strain. Quartz grains are sometimes enclosed in the felspar. The 
other minerals present are magnetite, in rudely-built crystals, and 
reddish isotropic garnet, often broken and slightly drawn out in the 
direction of foliation, 
[938] From the Lower Purple Clay, South of Withernsea. A 
compact white rock, enclosing black hornblende crystals, up to one- 
