106 Miss IW. K. Heslop—Pre-Tertiary Dyke, Usway Burn. 
and consist more of glass than of felspar, but the condition of the 
section does not necessarily indicate that of the whole crystal or 
erystalline group.’ 
3. The third set is characterized by deep zoning, sharp edges, few 
inclusions, and very indefinite twinning. Although distinct, these 
three types are probably due merely to different directions of the 
sections, or at most to slightly different physical conditions during 
erowth. They do not imply three generations of porphyritic 
felspars. 
The ground-mass felspars are broader and shorter than those which 
conmonly occur in the post-Tertiary dykes of Northumberland and 
Durham.? They show a combination of binary and multiple twinning, 
and on the whole are very free from inclusions, although they do 
sometimes contain rounded or oval patches of dark glass, and are 
pierced by apatite needles. They are very markedly zoned. 
The porphyritic pyroxenes are exactly like those which are described 
by Dr. Teall in the paper to which I have already referred. There 
he concludes that the prevailing pyroxene in the Cheviot Andesites 
and Porphyrites is not, as was previously supposed, augite, but 
hypersthene., 
In the pyroxene of the Usway Burn Dyke, sections which are cut 
perpendicular to the prism are octagonal, but the pinacoidal faces are 
largely developed at the expense of those of the prism (see Figs. 5 
and 6, Pl. XI). Cleavages parallel to both are usual, but that of 
the prism is the more distinct. These sections are decidedly pleochroic, 
the colour changing from yellow to reddish brown. They show 
straight extinction, and, in the vast majority of cases, a bisectrix in 
convergent light. Longitudinal sections, in which pleochroism changes 
the colour, from green for rays vibrating parallel to the length of the 
section, to yellow, for those vibrating at right angles to it, also show 
the point of emergence of a bisectrix, usually giving a much clearer 
figure than the cross-sections. They also show straight extinction. 
The cases in which an optic eye is obtained are various, but they 
generally have good traces of the brachypinacoids, and sometimes 
of the dome faces. 
A combination of these sections gives a crystalline form like that 
described by Dr. Teall as a ‘columnar doubly-terminated crystal, 
which is made up mainly of the pinacoidal and only to a slight 
extent of the prismatic faces’”’, while the consideration of the pleo- 
chroism and the interference figures shows that we are dealing with 
a crystal in which the axes of elasticity coincide with the erystallo- 
graphic axes, so we are justified in assuming that it is orthorhombic. 
That it is hypersthene and not enstatite or bronzite is shown by 
the strong pleochroism, and the constant occurrence of the clearest 
interference figure (bisectrix) in the least pleochroic sections (green 
and yellow), and the occurrence of a less definite figure in the normal 
1 Teall, Gnot. Mac., 1887, ‘‘ Notes on Cheviot Andesites and Porphyrites,”’ 
p. 160: “Tn this case the bulk of the foreign matter must be greater than that of 
the felspar substance, and yet the felspar has ‘impressed its character on the compound 
mass.’ 
2 Teall, Q.J.G.S., 1884, p. 229. 
