306 
HARKER : PETROLOGICAL NOTES. 
Micro. This is what some geologists would term a binary 
granite," though it has not the normal granitic structure. It consists 
almost solely of (quartz and felspar the quartz being of earlier con- 
solidation, in irregular rounded grains, moulded and enclosed by the 
felspar. The latter is opacj^ue and much altered, but appears to have 
been partly orthoclase, partly striated plagioclase. A few grains of 
magnetite occur. 
[941] From the Basement Clay at Bridlington Quay. A normal 
granite of medium grain. 
Micro. Both light and dark micas are abundantly present, 
sometimes with parallel intergrowth of the two. Felspar crystals 
often contain flakes of muscovite set parallel to the two principal 
cleavages, suggesting a secondary origin for this portion of the mica. 
The felspars are both orthoclase and an acid plagioclase. The latter 
has fine striation, the lamelliTc often bent and discontinuous, and 
frequently cross-twinning (the pericline law) in addition. The ortho- 
clase is often veined with albite (?) (' microperthite'). Clear quartz 
in grains of varying extent is the latest-formed constituent. 
No granite in the North of England has these characters, 
though rocks of this type occur in various parts of Scotland. 
[942] From the Lower Purple Clay, South of Witliernsea. A 
red granite, showing abundant pink felspar, colourless quartz, and 
dark mica (biotite). 
Micro. The deep brown biotite is often arranged interstitially 
to the felspar, an uncommon feature in such rocks. The felspar is 
partly orthoclase, partly striated oligoclase, but mostly microcline in 
broad crystal plates, showing in polarized light the most beautiful 
' cross-hatched ' structure, and often having the other felspar in shreds 
intergrown with it. The quartz, which has rows of crowded minute 
fluid-pores, is mostly later than the felspars, but occasionally included 
by them. 
[943] From the Lower Purple Clay, South of Withernsea. A 
biotite-granite, closely resembling the preceding, and having good 
thick flakes of dark mica. 
Micro. This sHde shows the same abundance of microcline, 
besides ' microperthite ' — orthoclase veined with albite in crystallo- 
