Miss M. K. Heslop—Pre-Tertiary Dyke, Usway Burn. 105 
minerals crystallized out in the following order: (1) and (2) apatite 
and iron oxide, (3) pyroxene, (4)felspar. The same order is preserved 
in the ground-mass generation, with the addition of a micaceous oxide 
of iron towards the close of the period. It is remarkable, however, 
that felspar laths of the normal ground-mass type are included in the 
porphyritic pyroxenes, and sometimes in the outer zones of the large 
felspars. At present there is no really satisfactory way of accounting 
for this apparent contradiction to the observed order of formation. In 
the last generation the elementary felspars take precedence of the 
pyroxenes, while much of the iron oxide still exists in an unindi- 
vidualized state. There is apparently some overlapping of the 
periods of formation, but the crystals are classed chiefly by their 
state of preservation and their size. 
The following figures give some idea of their relative dimensions. 
The first number in each bracket refers to the length, and the second 
to the breadth of the section. 
Porphyritie felspars (1:14 X *61mm.), (1:96 x -46mm.), (1°32 x 
‘70mm.) are of common occurrence, while (-43 x ‘28 mm.) is very 
small. 
Porphyritie pyroxenes, longitudinal sections: (1°4 X *d53mm.) is 
large ; (1°12 x :18 mm.) and (-96 X °50 mm.) are more usual; (°36 x 
-24mm.) is small. Some cross-sections are (‘76 X ‘71 mm.), (-39 X 
“32 mm.), (52 x 50 mm.). Longitudinal sections of the ground-mass 
felspars vary from (°34 X ‘18mm.), which is unusually large, to 
(035 x -0llmm.); the average size is about (12 X ‘03mm.). The 
largest cross-sections do not exceed (-12 X ‘21 mm.), and are generally 
yery much smaller. Longitudinal sections of ground-mass pyroxenes 
vary from about (-150 x 180mm.) to (051 xX 027 mm.). When 
smaller than this they are commonly included among the elementary 
forms. 
The porphyritic felspars may, for the convenience of description, be 
divided into three groups. 
1. In the first are those with sharp angles, and clear, well-defined 
edges (see upper phenocryst in Figs. 1 and 2, Pl. XI). They are 
characterized by a combination of simple twinning with that of the 
albite type, e.g. one-half of a crystal may be untwinned while the 
other shows the ordinary albite strie. Glass inclusions are common, 
but not conspicuous, and although traces of zoning have been observed, 
they are most indefinite, and entirely confined to the outer limits of 
the crystal. 
2. A second set comprises those felspars which have a deep 
peripheral zone of glass inclusions, bounded by a complete though 
narrow zone of felspar material. ‘These crystals are characterized by 
rounded angles and clear twinning of the albite type. They are 
usually made up of several particles, intergrown under plutonic 
conditions, apparently in some cases, then, as a group, furnished with 
a zone of glass inclusions and an outer felspar envelope, while at other 
times it is clear that these were added before the particles were 
grouped together, for each possesses complete zones of its own. 
Sections of these groups 7x the zone of inclusions are apparently 
riddled with glass (see lower phenocryst in Figs. 1 and 2, Pl. XI), 
