CARBONIFEROUS VOLCANIC ROCKS OF THE FIRTH OF FORTH BASIN. 497 
glass, but it gives the characteristic action of augite with polarised light. It is 
intimately mixed through the clear glass of the ground-mass, which it far 
exceeds in quantity. The iron oxides now appear asa fine granular dust, which 
is frequently aggregated into the elongated club-shaped objects just referred to, 
as if round some inner pellucid or translucent microlite. In patches throughout 
the field, however, the oxides take the form of a geometrically perfect network 
of interlacing rods. This beautiful structure, described and figured by ZIRKEL 
and others,* is never to be seen in any of the dolerites, except close to the line 
of contact with the surrounding rocks. It occurs also in some of the dykes. 
(See Plate XII, fig. 12). 
I have not succeeded in detecting any microlites in the sandstones at the 
edge of a dolerite sheet, though I have had many slices prepared for the 
purpose. The sandstones, so far as my observations go, do not offer any proofs 
of alteration capable of satisfactory elucidation by the microscope. 
Where dolerite has invaded sandstone there is usually a tolerably sharp line 
of demarcation between the two rocks. It is seldom easy to procure a hand- 
specimen showing the actual contact, for the stone is apt to break along the 
junction-line. Where, however, the rock traversed by the igneous mass is 
argillaceous shale, we may find a thorough welding of the two substances into 
each other. In such cases the dolerite at the actual contact shows a still further 
degree of diminution in its component particles. It becomes a dark opaque 
rock, which in thin slices under the microscope is found to be formed of a 
mottled or curdled segregation of exceedingly minute black grains and hairs in 
a clear glassy matrix, in which the augite and felspar are not individualised. 
But even in this tachylite-like rock perfectly formed and very sharply defined 
crystals of triclinic felspar may be observed ranging themselves as usual parallel 
to the bounding surfaces of the rock. These characters are well seen in the 
contact of the intrusive sheet of dolerite with shale and sandstone at Hound 
Point, described on p. 475. 
Another instructive example is furnished by the small threads which pro- 
ceed from the dolerite of Salisbury Crags, and traverse enclosed fragments of 
shale. Some of these miniature dykes are not more than 3th of an inch in 
diameter, and may therefore easily be included, together with part of the sur- 
rounding rock, in the field of the microscope. The dolerite in these ramifications 
assumes an exceedingly fine texture. The felspar is the only mineral dis- 
tinctly formed into definite crystals. It occurs in prisms, sometimes /th of an 
inch long, and therefore readily recognisable by the naked eye. These prisms 
are perfectly shaped, contain abundant twin lamelle, and show enclosures of the 
iron of the base. They had been already completely formed at the time of 
injection ; for occasionally they may be observed projecting beyond the wall of 
* Op. cit. p. 273 ; Vogelsang’s “ Krystalliten.” 
VOL. XXIX. PART I. 6M 
