148 THE BRITISH ISLES. 



continent, the Thames flowed eastward and formed part of the basin of the Rhine. 

 At that time it was merely a tributary river, but its volume was nevertheless far 

 more considerable than during a subsequent stage, when it flowed into a huge bay 

 of the sea, which reached up to London, and when the site of the great city was 

 occupied by an oyster bed.* At that period vast swamps extended to the eastward, 

 almost shut off" from the sea by a half-submerged littoral ridge, upon which, even 

 during post-tertiary ages, the bodies of huge animals floated down by the river 

 were stranded. The quantity of bones of rhinoceroses, mammoths, elephants, stags, 

 bisons, and other animals, which geologists have discovered in the marshes of 

 Ilfoi'd and elsewhere, is truly astonishing. At the present time the land once 

 more gains upon the sea, but this is due, in a large measure, to the work of man. 

 The sea-walls, perhaps commenced by the Romans, enclose an area of 33 square 

 miles, depressed between 3 and 7 feet below the level of high water.f 



At Teddington Lock, at an elevation of 21 feet above the level of the sea, the 

 Thames ceases to be an independent river. The tide flows up to that village, and 

 hence, perhaps, its name (Tide-end-ton J), but the river does not present the aspect 

 of an estuary until within a short distance of London, where muddy banks, 

 alternately covered and uncovered by the tide, are first met with. Even within 

 the limits of the metropolis the river frequently overflows its banks, and the 

 low-lying quarters to the south of it have more than once been invaded by its 

 floods. Yet in the basin of the Thames floods ought to be amongst the most 

 exceptional occurrences. § The rainfall is pretty regularly distributed through- 

 out the year ; there are no high mountain ranges bounding the basin ; the hills 

 within it are for the most part of gentle contours ; and the rain runs down slowly 

 from them into the river channels. As already remarked, the principal source, 

 near Cirencester, rises at an elevation of only 376 feet, but ^^rtually its surface is 

 about 30 feet lower, owing to its water being pumped into the summit " pound " 

 of the Thames and Severn Canal. But, besides this, more than one-half of the 

 basin of the Thames is composed of permeable rocks, which allow the water to 

 percolate into the bowels of the earth, instead of rapidly flowing down the hill- 

 slopes. The contrast between permeable and impermeable rocks strikes even the 

 superficial observer, permeable soil being planted with corn, whilst that which 

 retains the water is laid out in meadows. In the permeable district between 

 Nuneham and Maidenhead no tributary of any size enters the main river, and yet 

 it grows almost visibly with every one of its bends, owing to the numerous 

 perennial springs which rise on its banks. A régime such as this acts as a natural 



♦ Hugh Miller, " Summer Ramble among the Hebrides." 



t Redman, Institution of Civil Engineers, 1877. 



t Huxlej-, " Physiography." 



§ Volume of the Thames at Teddingfon Lock : — 



A vcrage discharge, per second 1,300 cubic feet. 



jMaximum „ „ 1,770 „ 



3Iinimum „ „ 700 „ 



Area of the basin above Teddington Lock . . . 4,590 square miles. 



Rainfall within the basin ....... 26 inches. 



Surface drainage 4 „ 



