198 GRAVEL OF CAMBRIDGE, AND OF VALLEY OF THE THAMES. 
of the neighbouring rocks of oolite and chalk, with an occasional ad¬ 
mixture of quartz and other pebbles from the central counties. The 
latter occur also in Whittlebury Forest near Northampton, and at 
Brackley. The late Sir Joseph Banks informed me he had observed 
pebbles of porphyry in the road gravel on the north side of the town 
of Dunstable. There is no nearer place from which these latter could 
have been derived than the porphyritic rocks of Charnwood Forest, 
in Leicestershire. 
Professor Sedgwick has ascertained, that the gravel beds on the 
summit of the Gogmagog Chalk Hills, near Cambridge, and on the 
hills adjoining towards Bedfordshire, as well as that in the valleys, 
contain not only the wreck of chalk strata, but also fragments of 
almost every formation that occurs in England; amongst them he has 
found the joint of a basaltic pillar, between one and two feet long. 
Another striking example of a similar kind is afforded by the 
gravel of the valleys of the Thames from London to Oxford, and of 
the Cherwell and Evenlode, that fall into the Thames from the 
northern parts of Oxfordshire. (See Plate XXVII.) I shall subjoin, 
in an appendix, a detailed account of this gravel, and of the state of 
the hills and valleys over which it is dispersed, extracted from a paper 
I have published on the Lickey Hill, in the 5th vol. of the Geological 
Transactions. Its phenomena are in perfect unison with all the other 
cases I have been examining, and show the effect of a violent rush of 
waters from the north, which has drifted pebbles of quartz rock from 
the plains of Warwickshire, and other central counties, over the whole 
country intermediate between them and London, along the line of 
these three rivers; and has mixed them up in each district with the 
