TATE: YORKSHIRE PETROLOGY. 
the crystalline faces of hornblende, and may be seen in the upper 
corner of figure 24. But the field is sprinkled over with the 
iron-ore from this source, in the form of opaque grains and crystals 
of magnetite and titanoferrite. 
At one point what seems to have been originally a pocket of 
hornblende has been converted into brown mica and sphene. The 
latter mineral occurs in well-formed crystals and plates of a golden- 
brown tint, by transmitted, reddish b}" reflected light. Usually two, 
if not more, of the crystalline faces are sharply defined by dark 
borders, and their surfaces are very much pitted (figs. 37-38). The 
mineral is pleochroic, polarising in moderate tints. An opaque patch 
of titanoferrite, not yet transformed into sphene, but with the altera- 
tion product, dark by transmitted, white by reflected liglit 
(leucoxene) coating its interior, lies in one corner of figure 37. 
The mica plates in this inclusion are ver}' pale-brown, and have a 
peculiar labyrinthic structure (fig. 38). In another case, a cavity 
apparently vacated by a hornblende crystal, is filled in with lath- 
shaped plates of an indeterminate mineral, brown by transmitted, 
salmon-tinted by reflected light, radiating trom its internal sur- 
face : the clear interspaces to polarise feebly (fig. 30). Strongly 
polarising calcite plates and powder, or a feebly dichroic and 
polarising ])ale-green substance of the cliloritoid family, fills in 
many of the cavities vacated by felspar oi- hornblende crystals. To 
these must be added a few blebs of pyrites, and, more sparsely, 
secondary quartz to complete our micro-petrographical description. 
Sharply contrasting with the above dyke is the lowermost one of 
the gi'oup, which stands out boulder-like from the surrounding gTass 
some hundred and twenty yards or so higher up the stream. The 
rock, of which not more than two or three cubic feet are exposed, 
weathers to a Avarm brown tint, a freshly fractured face having a 
steel-grey hue, glistening with densely-massed small hexagonal plates 
of browm mica, this being the only recognizable mineral in a hand- 
specimen, save here and there a porphyi'itic crystal of pink felspar. 
By transmitted light the rock exhibits a hypo-crystalline struc- 
ture, devitrification of the glassy magma, having developed doubly 
refractive prismatic felspar microlites, whose fan-like groupings, with 
