Arecs of Drainage. 495 



on the west produce a like effect ; and thus, partly 

 because it is the first land that the wind laden with 

 moisture reaches, and partly because of the mountains, 

 it happens that a greater amount of rain is precipitated 

 in the western than in the eastern parts of our Island. 



If we examine our country with regard to special 

 areas of drainage, we find, that they are exceedingly 

 numerous. In Scotland the rivers that run into Moray 

 Firth drain an area of about 2,500 square miles ; the 

 Spey, which runs into the German Ocean, nearly 1,200 

 square miles. The Tay drains an area formed by the 

 Grampian mountains and part of the Old Eed Sand- 

 stone of 2,250 square miles. The Forth, including its 

 estuary, drains an area of about 2,000 square miles. 

 The Clyde, not including the greater part of its estuary, 

 drains an area of 1,580 square miles, the Tweed 1,870 

 square miles. 



In England, the Tyne drains 1,100 square miles, 

 the Tees, 774. If we take the Trent and the Ouse as 

 draining one area, the immense extent, for such a 

 country as ours, of about 9,550 square miles are drained 

 into the Humber. The Witham, the Welland, the 

 Nen, and the Great Ouse, flowing into the old bay of the 

 Wash, drain 5,850 square miles. The Thames drains 

 an area of about 6,160 square miles ; and if we include 

 all the estuary, about 10,000. The Severn drains an 

 area of 8,580 square miles. The Avon that enters the 

 sea at Christchurch drains 1,210 square miles ; the Ex, 

 643 ; the Towey, in Caermarthenshire, 506 ; the Dee, 

 862; the Mersey, 1,748; the Kibble, 720; and the 

 Eden, 995 ; and if we take all the rivers that run into 

 the Solway Firth, including the Eden, the area drained 

 amounts to nearly 3,000 square miles. This leads to 

 the question of the origin of river valleys and their 

 different geological dates. 



