INTRODUCTION. Xx1 
At Binfield Heath and Caversham they cover more than a square 
mile; in the latter place the London Clay is brought in by two 
faults at its eastern end. It consists almost wholly of bluish 
grey clay, in which are septaria, and is much covered by gravel. 
These outliers of the Tertiary are very important factors in 
Botanical distribution, the stiff clay holding up the water in 
small ponds, in which Peplis, Helosciadium inundatum, Littorella, 
Ranunculus truncatus occur, and when covered as at Binfield 
with sandy gravel, afford a home for Centunculus, Radiola, 
Scutellaria minor, Ulex nanus, Erica tetralia, L. cinerea, Nardus 
stricta, Aira flexuosa, etc. 
The Drifé has already been frequently mentioned. To the 
occurrence of this in the form of the high level gravels the 
existence of our ericetal flora is especially due. The low level 
gravels have been formed by the present rivers cutting their 
way and reassorting the older drift pebbles. 
The Alluvial deposits may be seen by the Windrush river 
from Witney to its junction with the Isis, where they are 
nearly a mile wide, and also in the rich water-meadows of the 
Cherwell and Thames, Port Meadow being a good example. 
Caltha palustris, Cardamine amara and pratensis, with the 
Fritillaria, are especially noticeable features of their flora. 
DRAINAGE. 
Oxfordshire is almost entirely included in the Thames 
basin, the area of which, according to the report of the Rivers 
Commission, is about 5162 square miles, The greatest length 
from Trewsbury mead to the estuary of this basin is 201 miles: 
its greatest breadth is from Priors Marston in Warwick to Fern- 
burst in Sussex. It includes besides Oxfordshire the whole or 
nearly the whole of Berks, Bucks, Herts, Middlesex, Surrey, a 
third of Gloucestershire, a fourth of Essex, a sixth of Wilts and 
Kent, and small portions of five other counties. 
The Thames rises on the south-eastern slope of the Cots- 
wolds, reaches Oxfordshire at Kelmscott near Lechlade, and 
forms the county boundary between Oxford and Berks, thence 
to Henley, receiving in its course to Wolvercote the Charney 
brook, the Windrush, and the Evenlode. 
