494 PROFESSOR GEIKIE ON THE 
appear they are small in size and limited in their range through the rock. It 
is distinguished by a peculiar uniformity of texture, seldom showing any large 
porphyritic crystals, such as frequently occur in the basalts. In some rare 
cases it may be columnar, but the columns are large and rude when compared 
with those of the basalts. The edges of a sheet where it approaches the 
contiguous strata present a much finer grain than the central portions. Small 
veins of this compact variety may occasionally be found penetrating the 
adjoining rocks, which are usually much indurated, both above and below 
intrusive sheets. 
Rocks presenting these characters abound in this region; the great sheets 
traversing the Lanarkshire and Stirlingshire coal-field, those in the heart of Fife, 
and many of those in the West of Mid-Lothian may be taken as illustrations. 
2. Microscopic Characters.—As above remarked, no sharp line can be 
drawn between these rocks and the diabases already described. The dolerites, — 
however, seldom contain any orthoclase or any original quartz, and are never 
so coarse in texture. They show a remarkably crystalline structure under 
the microscope. In many varieties no distinct trace of any glassy ground-mass 
can be detected. In others this substance appears as a clear pale yellow or 
green limpid glass, crowded with dark trichites, and. sometimes with minute 
green transparent microlites and needles of apatite. 
The felspar is triclinic, and appears in clear colourless striated prisms some- 
times half an inch long. It is probably labradorite ; it can at least be easily 
distinguished from the milky translucent variety so common among the 
diabases. It sometimes contains minute glass enclosures, and is often studded 
with apatite needles, or crossed by stouter prisms of that mineral. Small dark 
grains, which may be titaniferous iron or magnetite, may likewise be observed. 
Not infrequently a remarkable finely fibrous, transparent, or translucent 
substance may be observed encrusting the felspar. It resembles the occurrence ; 
of a zeolite. Occasionally a detached portion of augite may be detected entirely 
enclosed in the clear felspar. But there can be no doubt that the latter 
mineral was the first to crystallise into definite prisms. Traces of orthoclase 
are comparatively infrequent, but a clear or a kaolinized prism twinning in the 
vertical axis may now and then be detected. 
The augite occurs in large well-defined crystals, in irregular kernels, and 
rarely in minute granules. In most of the dolerites, particularly in the coarser 
varieties, it presents a fractured or flawed structure as in the diabases, and not — 
uncommonly it might be supposed to have been penetrated across its figures 
by intrusive prisms of felspar. Beautiful examples of this relation of the two — 
minerals may be noticed in the dolerites of the Falkirk and Slamannan coal-field. 
Examination with polarised light, however, shows that the apparent dislocations 
