G-EOLOGY AND SUEFACE FEATURES. 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 

 round 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. Similarlj'-, 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 with 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 and Surface Features. 

 England is distinguished among all the countries of Europe for its great variety 

 of geological formations. It is the very 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 country 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 a 



* Table of British Formations, according to Professor A. C. Eamsay : — 



Eecent Alluvia, Peat, and estuarine beds now forming, &c. 



Post Tertiary River and estuarine alluvia ; glacier moraines and boulder 



clays ; forest bed of Norfolk. [Tertiary 



