208 Physical Geography. 



orum. Interstratified with the Weald Clay there are 

 a few thin bands sparingly charged with the remains of 

 marine shells. 



Enough has now been said to prove the fresh-water 

 and estuarine character of the Purbeck and Wealden 

 beds, and also, considering the broad spread of these 

 formations in England, that they must have been depo- 

 sited near and at the mouth of a large river. But to 

 estimate the possible dimensions of this Delta we must 

 go further afield. 



It has been customary to estimate the area occupied 

 by these deposits by measuring their length from west 

 to east, between the Vale of Wardour and the Boulonnais 

 in France, and from north-west to south-east, from Hamp- 

 shire to Vassy, or in some cases taking a shorter di- 

 ameter to Beauvais, and the respective diameters given 

 of these lines are in the first case 320 miles and in the 

 second 200 miles. 1 Even if these measurements were 

 correct, which they are not, this method seems to me 

 to be erroneous, for the measured diameters run too 

 much in the same direction, whereas, as much as possible, 

 they ought to be measured at right angles to each other. 

 The real measurement from west to east, between the 

 Vale of Wardour and the Boulonnais, is about 200 

 miles, and a line drawn nearly at right angles to this, 

 between the south side of the Isle of Wight, where the 

 Weald Clay occurs, and Quainton, in Buckinghamshire, 

 where we find the most northerly outlier of the Purbeck 

 beds, is about 100 miles in length. This would give an 

 area for the Delta of about 20,000 square miles. 



Kigidly to adhere to this measurement, as an accu- 

 rate account of the size of the ancient Delta, would, 



1 See Lyell's Student's Elements of Geology,' p. 304, second 

 edition. 



