The universal geography : earth and its inhabitants

50
THE BEITISH ISLES.
80 feet, are being worked, and the quantity of coal whicli it is possible to extract
without descending to a greater depth than 4,000 feet is estimated by Vivian
and Clark at more than 36,000,000,000 tons. In the west the seams yield
anthracite, but in proportion as we proceed eastward the coal becomes more and
more bituminous, the gases enclosed in it often giving rise to fearful explosions,
the frequent recurrence of which is a calamity which might generally be obvi-
ated by judicious cautionary measures. So fiery is some of this "Welsh coal, that
after having been placed on shipboard it will ignite spontaneously.
The researches of men of science have conclusively proved that "Wales, within
recent geological time, has undergone variations of level. Marine sheUs of living
species were discovered as long ago as 1831 near the summit of Moel Tryfaen,
^1f
Fig. 23.
^B.-
—
Erosive Action on the Coast or Soitth Wales.
Scale 1 : 100,000.
to the south of the Menai Strait, at an elevation of 1,400 feet above the
level of the sea. This discovery has been confirmed and followed up by other
geologists, including Edward Forbes, Prestwich, Ramsaj^ Darwin, and Lyell.
Mr. Darbishire has found fifty-seven marine molluscs in the upheaved strata
which during the post-pliocene epoch formed the beach, and all these shells belong
to species which stiU live in the neighbouring sea or in the Arctic Ocean. The
general character of this ancient fauna points to a climate as rigorous as that of
Iceland or Spitzbergcn. The British seas were colder at that time than now,
and when the land once more emerged from the sea these shell banks became
covered with the detritus brought down by glaciers.*
* LyoU, " Elements of Geology.''