Miss M. K. Heslop—Pre-Tertiary Dyke, Usway Burn. 107 
transverse sections—in other words, the acute bisectrix coincides 
with the brachydiagonal and the crystals are negative. These are 
the characteristics of hypersthene. 
Dr. Teall claims that a monoclinic pyroxene is also present in 
the Cheviot igneous rocks, and mentions twinning (parallel with 
the pinacoids) which, he suggests, only takes place in the mono- 
clinic mineral. Twinning parallel both to the prism and pinacoids 
may be seen in the Usway Burn pyroxene, but it is found sometimes 
in crystals which are undoubtedly orthorhombic. There are, however, 
certain sections noticeable in ordinary light for their irregular outlines, 
deep cleavage, and dark colour—but little changed by pleochroism— 
and between crossed nicols for their brilliant interference colours 
and frequent twinning. There can be little doubt that they are 
monoclinic crystals of augite. Sections perpendicular to the prism 
give, in convergent light, a dark arm surrounded by coloured bands. 
Sections parallel to the prism give various and somewhat doubtful 
figures, never a good bisectrix. 
In a much decomposed specimen of the rock, all the large well- 
formed pyroxenes are altered to colourless masses streaked with 
green fibres of chlorite, and including little dark rods. Patches of 
the original mineral may sometimes be found towards the centres 
of these crystals, and in one or two cases the fresh portion was 
distinctly twinned. It would be difficult to say, in these circum- 
stances, whether the fresh portion is orthorhombic or monoclinic, 
but that the whole crystal was originally orthorhombic is beyond 
question. In this altered specimen the only wholly fresh crystals 
are those described above as augite, and although some orthorhombic 
sections are only partially decomposed, they belong to the ground-mass 
generation. It would seem, then, that both orthorhombic and mono- 
clinic pyroxenes are present, but that the former is either older or 
more readily decomposed than the latter; also that twinning is common 
to both, and is probably, in the orthorhombic mineral, a precursor of 
decomposition, perhaps of optical re-orientation. 
It is difficult to draw a definite line between the porphyritic and 
ground-mass pyroxenes. The latter, unlike the felspars of their own 
generation, occur in exactly the same way as their porphyritic 
equivalents. They are always surrounded by a little halo of pale 
yellow material with which the incomplete crystals (of common 
occurrence) are intergrown in a way that recalls the micro-pegmatitic 
intergrowth of quartz and felspar (see photographs 5 and 6). There 
is no clue, however, to the identity of the intergrown substance in 
the case we are considering, for although it is not absolutely isotropic, 
its interference effects are too feeble to be of any use. These in- 
complete crystals may be correlated with the incomplete prisms of 
the third generation, a description and suggested explanation of 
which is given later. 
In a decomposed specimen of rock, the pale zones surrounding 
altered pyroxenes, both large and small, become very conspicuous, 
and contain tiny prisms of the fresh mineral, lying with their 
axes parallel to those of the large crystals to which they are 
attached. 
