The universal geography : earth and its inhabitants

GEOLOGY AXD SUEFACE FEATUEES. 7
Britain had been severed from the neighbouring continent. Yet, compared with
the open Atlantic, or even with inland seas in volcanic regions, its depth is incon-
siderable. Only detached portions of its bed sink below 300 feet, and the
maximum depth does not exceed 500 feet. In the North Channel, however,
the depth is greater, being nowhere less than 300 feet, and attaining 664 feet
in one of the " pits " lying midway between Galloway and the Lough of
Belfast.
The tidal undulation reaches the British Islands from the south-west, and,
travelling along the west coast of Ireland and Scotland, wheels completely
roimd the north of the islands, so that the old tide coming from the northward,
down the German Ocean, meets the Atlantic tide of twelve hours later date
opposite to the mouth of the Thames. Similarly, opposite tidal currents pene-
trate into the Irish Sea from the north and the south, meeting about the
parallel of the Isle of Man. The rise of the tide is generally greater on the
exposed west coast than on either the south or east coast, but varies exceedingly
according to local circumstances. "Where tidal waves meet, a higher rise is the
result, but where the time at which a high tide wave reaches a particular coast
coincides wit"h the moment of ebb of a tidal wave coming from another direction,
the two undulations neutralise each other. Thus, on the south-east coast of
Ireland, and at the Portland Bill, in the English Channel, the two undulations
almost balance each other, and the tide is consequently hardly perceptible. On
the contrary, when the tidal wave enters a narrowing arm of the sea or an
estuary, it advances with increasing impetuosity, and attains a considerable
height. The most conspicuous instance of this is presented by the Bristol
Channel, which becomes shallower as it narrows, and where the spring tides con-
sequently attain a height of 60 feet. The general rise of the tides, however, is
far less.
Geology A^D Surface Featuees.
England is distinguished among all the countries of Eiu-ope for its great variety
of geological formations. It is the veiy paradise of geologists, for it may be
said to be in itself an epitome of the geology of almost the whole of Europe, and
of much of Asia and America. There are few formations which are not repre-
sented at least by a few patches, and so regular is their succession that the geology
of England, in its general features no less than in its details, became sooner known
to us than that of any other coimtry in Europe. The geological map which
William Smith published in 1815, after twenty-five years of unwearied work, in
the course of which he traversed England on foot in all directions, is a remark-
able work, and surprises by the relative perfection with which it brings to our
knowledge the extent of the various geological formations.* Since his time u
Table of British Formations, according to Professor A. C. Eamsay :
Recent Alluvia, Peat, and estuarine beds now forming, &c.
Post Tertiary Eiver and estuarine alluvia ; glacier moraines and boulder
—
clays ; forest bed of Xorfolt. [Tertiary