376 
TATE: YORKSHIRE PETROLOGY. 
taken across the cleavage being a little paler: both arj strongly 
dichroic. The plates are very free from microscopic enclosures, but 
numerous transparent acicular microliths and globnlites, following 
three directions crossing at an angle of 60®, are revealed by higher 
magnification (Fig. 6). Where disintegration has set in, e.^., slides 
cut near the exposed surface, the biotite is spotted with ferric oxides 
arranged often parallel with the cleavage planes, or symetrically 
within the basal plates (Figs. 7 and 11). Further, the plates are 
occasionally bent and driven apart, with their ends frayed out, 
where the supporting felspar or hornblende has been dissolved out. 
Here we see the earlier stages in the disintegration of this secondary 
biotite (Figs. 12 and 13). The clusters of felspar crystals enclosed 
by mica, noticed in the hand specimens, have here also representa- 
tives of microscopic dimensions (Fig. 15). An incipient foliation is 
at some points simulated by the biotite plates, showing a tendency 
to arrange themselves parallel with the margin of the dyke, as 
*though the shearing stress had here been more intense. Magnetite 
and apatite being the first to crystallize, are included within all the 
later-formed minerals — the former appearing as octohedral plates, or 
granular patches (Fig. 21), and the latter in sharply defined colourless 
hexagonal prisms. Much of the magnetite, however, in these 
sections, especially if cut from near the exposed face of the rock, 
appears as secondary skeletal crystals (Figs. 12 and 14), or irregular 
plates having their margines converted into translucent red haema- 
tite (See Fig. 1), or opaque brown flecks of limonite. Sparsely 
distributed rhombohedral plates or grains of titaniferous iron, 
occasionally surrounded by an opaque border of leucoxene are also 
usually associated, as is the magnetite, with an altered condition of 
the biotite as previously described (Figs. 1 and 12). Among the 
secondary products of degradation may be noted quartz in clear 
colourless crystals, or crystalline grains ; scales of kaolin opaque 
by transmitted, white by reflected light ; a very pale green mineral 
indefinite in outline, not dichroic and not polarising ; dolomite in 
polygonal grains or scattered dust strongly chromatic (rich green 
and pink tints under crossed Nicols) ; and specks of pyrites. Lastly, 
calcite, as an infiltration from the overlying carboniferous limestone, 
