coast as at Carrick-a-raide. The streams also brought down
sand and gravel from the uprising domes of trachyte, and deposited them
over the lake-bed along with the erupted ashes.[4] The epoch we are now
referring to was one of economic importance; as, towards its close,
there was an extensive deposition of pisolitic iron-ore over the floor
of the lake, sometimes to the depth of two or three feet. This ore has
been extensively worked in recent years.

[Illustration: Fig. 30.--Cliff section above the Giant's Causeway, coast
of Co. Antrim, showing successive tiers of basaltic lava, with
intervening bands of bole.]

(_f._) _Fourth Stage (Volcanic)._--The last stage described was brought
to a termination by a second outburst of basic lavas on a scale probably
even grander than the preceding. These lavas consisting of basalt and
dolerite, with their varieties, and extruded from vents and fissures,
spread themselves in all directions over the pre-existing lake deposits
or the older sheets of augitic lava, and probably entirely buried the
trachytic hills. These later sheets solidified into more solid masses
than those of the second stage. They form successive terraces with
columnar structure, each terrace differing from that above and below it
in the size and length of the columns, and separated by thin bands of
"bole" (decomposed lava), often reddish in colour, clearly defining the
limits of the successive lava-flows. Nowhere throughout the entire
volcanic area are these successive terraces so finely laid open to view
as along the north coast of Antrim, where the lofty mural cliffs, worn
back into successive bays with intervening headlands by the
irresistible force of the Atlantic waves, present to the spectator a
vertical section from 300 to 400 feet in height, in which the successive
tiers of columnar basalt, separated by thin bands of bole, are seen to
rise one above the other from the water's edge to the summit of the
cliff, as shown in Fig. 30. Here, also, at the western extremity of the
line of cliffs we find that remarkable group of vertical basaltic
columns, stretching from the base of the cliff into the Atlantic, and
known far and wide by the name of "The Giant's Causeway," the upper ends
of the columns forming a tolerably level surface, gently sloping
seawards, and having very much the aspect of an artificial tesselated
pavement on a huge scale. A portion of the Causeway, with the cliff in
the background, is shown i