178 SOUTH COUNTRY TROUT STREAMS 



but covered with alluvium. Advancing westwards into 

 Dorsetshire the ooHtes appear, but they occupy only a 

 small breadth of country. From the Isle of Purbeck, 

 where the upper wealden beds are found, to Portland 

 Island, where the upper oolites are developed and yield 

 a valuable building stone, the distance is very small, and 

 from Portland Bill to Lyme Regis, where the lias comes up 

 from beneath the oolites, it is also inconsiderable. The 

 new red sandstone, which then succeeds, is of greater 

 breadth, but by far the most completely developed deposits 

 are those still further to the west and much older, belong- 

 ing to the Devonian period, the intervening carboniferous 

 series being abundantly but not characteristically repre- 

 sented. Granite bases have brought up these rocks and 

 the surface has been subsequently denuded. It is only 

 in the northern parts of Devonshire and in parts of Corn- 

 wall that the slates and shales of these ancient periods 

 come to the surface ; but there they entirely replace the 

 more modern deposits. The nature of the rocks has a 

 marked influence on the quantity as well as the quality 

 of the waters that run off the surface, as the physical con- 

 dition of the surface and its orography influence the 

 quantity of rain that falls in the district. 



" 3. Siib-divisiofi of the District. — The whole district 

 naturally divides into two. The eastern portion extends 

 from the South Foreland to the western watershed of the 

 Hampshire and Wiltshire Avon, and includes the country 

 south of the Thames basin. This juirt is the smallest, 

 has the fewer streams, and the smaller rainfall, with the 

 exception of the drainage area of the Stour, which is the 

 western branch of the Avon ; the rivers are all short, 

 commencing only a few miles back from the sea. Tiie 

 western portion of this extensive district has a much 

 larger surface, many mon; streams, higher elevation, and 

 a heavier rainfall ; but the streams are scarcely more 

 important, and none of them possess more than local 

 interest. The western group of streams is again sub- 

 divided into two, those which drain southwards to the 

 English Channel, and those which empty themselves into 

 the Bristol Channel flowing towards the north. 



"4. Sources of Water Sup ply.--' Y\iQ eastern rivers of 

 this district have their sources m the wealden rocks, or 



