The core composition for all three types of crystals, as determined mainly by extinction angle methods, is bytownite for units A, C, E and G; labradorite for unit B and oligoclase for unit D.
Unit A is the most abundantly porphyritic basalt unit and its resultingly high content of bytownite is reflected in its high [A1.
In vogesite its composition ranges from andesine to
bytownite, and minor albite occurs in the groundmass of monchiquite (Lindgren and Ransome, 1906; Loughlin and Koschmann, 1935; Thompson, 1992).
