n the figure (Fig. 31). The columns are
remarkable for their symmetry, being generally hexagonal, though
occasionally they are pentagons, and each column is horizontally
traversed by joints of the ball-and-socket form, thus dividing them into
distinct courses of natural masonry. These are very well shown in the
accompanying view of the remarkable basaltic pillars known as "The
Chimneys," which stand up from the margin of the headland adjoining the
Causeway, monuments of past denudation, as they originally formed
individuals amongst the group belonging to one of the terraces in the
adjoining coast.[5] (Fig. 32).

[Illustration: Fig. 31.--The Giant's Causeway, formed of basaltic
columns in a vertical position, and of pentagonal or hexagonal section;
above the Causeway is seen a portion of the cliff composed of tiers of
lava with intervening bands of bole, etc.--(From a photograph.)]

[Illustration: Fig. 32.--"The Chimneys," columns of basalt on slope of
cliff overlooking the Atlantic, north coast of Co. Antrim. The
horizontal segments, or cup-and-ball joints, of the columns are well
shown in this figure. (From a photograph.)]

(_g._) _Original Thickness of the Antrim Lavas._--It is impossible to
determine with certainty what may have been the original thickness of
the accumulated sheets of basic lavas with their associated beds of ash
and bole. The greatest known thickness of the lower zone of lavas is, as
I have already stated, about 600 feet. The intermediate beds of ash and
bole sometimes attain a thickness of 40 feet, and the upper group of
basalt about 400 feet; these together would constitute a series of over
1,000 feet in thickness. But this amount, great as it is, is undoubtedly
below the original maximum, as the uppermost sheets have been removed by
denuding agencies, we know not to what extent. Nor is it of any great
importance. Sufficient remains to enable us to form a just conception of
the magnitude both as regards thickness and extent of the erupted matter
of the Miocene period over the North-east of Ireland and adjoining
submerged tracts, and of the magnitude of the volcanic operations
necessary for the production of such masses.

(_h._) _Volcanic Necks._--As already remarked, no craters of eruption
survive throughout the volcanic region of the North-east of Ireland,
owing to the enormous extent of the denudation which this region has
undergone since the Miocene Epoch; but the old "necks" of such
crater