CHAP. XXVII 
453 
SILLS, BOSSES AND DYKES OF THE BUYS 
amorphous patches ot titaniferous iron or magnetite give place to minute 
particles, whicli tend to group themselves into long cluh-shaped bodies. Ihe 
labradorite continues but little affected, except that its prisms, though as 
defined, may not be quite so large. The interstitial glassy groundmass remains 
in much the same condition and relative amount as in the centre ol the rock. 
Along the line of contact, wliile the dolerite becomes exceedingly close- 
grained, its felspar crystals are still quite distinct even up to the very edge. 
But they become fewer in relative numlier, and still smaller in size, though 
an occasional prism two or three millimetres in length may occur. They 
retain also their sharpness of outline, and their comparative freedom from 
enclosures of any kind. They tend to range themselves parallel with the 
surface of the contact-rock. The augite exists as a finely graiudar pale green 
substance, which might at first be taken for a glass, but it gives the charac- 
teristic action of augite with polarized light. It is intimately mixed through 
the clear glass of the groundmass, which it far exceeds in quantity. The 
iron oxides now appear as a fine granular dust, which is frequently aggre- 
gated into elongated club-shaped objects, as if round some inner pellucid or 
translucent mierolito. 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 ot contact with the 
surrounding rocks. It occurs also in some ot the dykes. I ha's e 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. 
AVhere one of the dolerite sills has invaded sandstone, there is usually 
a tolerably sharp line of demarcation between the two rocks, though it is 
seldom easy to procure a hand-specimen showing the actual contact, for the 
stone is apt to break along the juncthm-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 becomes a dark opaque rock, which in thin slices under 
the microscope is found to be formed ot 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 individualized. But even in this 
tachylyte-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 tlie intrusive sheet of dolerite with shale and sandstone at Hound 
Point (Fig. 160). 
Another instructive example is furnished by the small threads which pro- 
ceed from the dolerite of Salisbury drags, and traverse enclosed fragments of 
shale (Fig. 162). Some of these miniature dykes are not more tlian one- 
eighth of an inch in diameter, and may therefore easily be included, together 
with part of the surrounding rock, in the field of the microscope. The dolerite 
in these ramifications assumes an exceedingly fine texture. The felspar is the 
^ Uikroskoinsclm Beschaffenlieii dev Mmeralien und GesUine, p. 273 ; \ ogelsaiig’s Krystalliten. 
