46 PHYSICAL GEOGRAPHY. CHAP. 



of the land was wasted, during the long periods while the depths 

 of the sea were such as to permit of the effect of breakers on the 

 shore and currents on the bed. Both while rising and while falling, 

 the water hammered against the shores and dredged along the 

 channels ; wasting the surface, reducing the heights, digging out 

 the valleys, and spreading detritus over submarine plains. 



Following continually the retiring sea, rivers often swept away 

 the traces of its action, or covered them with fresh deposits. At- 

 mospheric vicissitudes, rains and snows, heat and cold, disintegrated 

 the rocks ; carbonic acid aided in dissolving them ; new phenomena 

 replaced the older ones, new features were impressed on every hill 

 and every hollow; and thus our land surface, as we see it, exhibits 

 in every part the modifications produced by what may be called 

 the c ordinary action' of daily causes, these being superimposed 

 on broader and greater features generated by elevation and de- 

 pression on a great scale, accompanied by powerful waves and 

 strong currents of the sea. 



Supply of Water in the Thames. The drainage area of the Thames 

 above Oxford (exclusive of the Cherwell) is about 600 square miles. 

 The rainfall may be taken at i\ feet in depth in a year. The total 

 quantity of cubic feet falling on this surface in a year 



= aj x 600 x 528o 2 = cubic feet 41,817,500,000; 

 and in a minute = cubic feet 79j57' 



The dry-weather summer-flow, being observed at Wolvercot and 

 Wytham, = 11,620 cubic feet in a minute; this is proportioned to 

 4-38 inches out of 30 inches. 



The dry- weather winter-flow (without rain), being noted at the 

 same place, = 22,624 cubic feet; this is proportioned to 8^55 inches 

 out of 30 inches. 



The flow in rainy weather being taken at 35,498 cubic feet; this 

 is proportioned to 13*35 inches out of 30 inches. 



And the floods being estimated at double the rainy weather 

 quantity, = 70,996 cubic feet; this is proportioned to 26-70 inches 

 out of 30 inches. 



The summer and winter measures of the perennial spring- water 

 give us an average 6*46 inches out of 30 inches. 



If moderate rainy weather be supposed to occur for one-fifth of 



the year, this will add to the above -A^P t =1*38 inches ; 



