TATE: YORKSHIRE PETROLOGY. 
375 
flesh-coloured crystals ; each cluster lying within an envelope of 
black mica, which, in optical transverse section, forms a border or 
frame to the brighter coloured crystals within. The crystalline 
structure is finer near the margins of the d5^ke than in the middle 
where the nests become conspicuous. A few blebs of iron-pyrites 
shine out from the exposed face. The joints are few in number, 
dividing up the dyke into bold cubical masses. "Weathering produces 
a light-brown earthy face, in which the original mineral constituents 
are no longer distinguishable. 
Microscopically, the most prominent features on a polished face 
are the sections of hornblende, these in all cases being best seen by 
reflected light. They are of a pale greenish-grey tint, either 
hexagonal sections parallel with the basal plane, or longitudinal 
sections parallel to the face of the long prism (Fig. 3). By trans- 
mitted hght (Fig. 1), the notable character is the entire absence of a 
glassy base, the whole of the component materials being well 
crystallised out. The orange-red felspar crystals are very uniform' 
in size, with their rectangular boundaries sometimes fairly well 
defined. They yield the extinction angles of orthoclase, but 
generally the crystals are much altered by the development, 
especially when in contact with hornblende, of microfelsitic matter 
polarising (Fig. o) in blue-milk tints, and of a faint greenish- white 
colour by reflected light. With higher powers these decomposition 
products 8-re seen to follow the lines of cleavage (Fig. 4). It is 
proposed to recur to this characteristic structure when dealing with 
the incipient schillerization to which some of the ingredients of these 
Mica-traps have been subjected. The hornblende crystals have clear 
boundaries, within which the felspar never intrudes. The crystal- 
lization of the hornblende preceded that of the felspar. The 
characteristic cleavages of hornblende are only faintly indicated in 
transverse sections of this nearly colourless type, which is no longer 
dichroic ; nor are the usual fibrous striations to be detected in the 
longitudinal sections. The mica, which is plentiful, and remarkably 
fresh, is the uniaxial biotite, giving the maximum of extinction 
between crossed-Nicols when viewed at right angles to the basal 
plane. The hexagonal plates are olive-brown, transverse sections 
