Page:Transactions of the Geological Society, 1st series, vol. 3.djvu/252

 a manifest alteration, its high elevations subsiding by degrees into the sand hills of the latter.

Upon some of the highest hills near Cambridge, a deposit of gravel and loose stones in horizontal layers, has lately been found, resting immediately upon the chalk. This gravel differs in so many respects from the red ferruginous gravel found dispersed in patches over the gault in the subjacent flat, that I think it must be considered as a deposit of a different epoch. It contains numerous fragments of strata belonging to the oolite series, which occur in the neighbouring counties of Northampton and Rutland, surrounding Cambridge on the west and north-west. Pieces of basaltic rocks are sometimes found, but these are not very common. These fragments are of all sizes, and worn down in different degrees. Some are pebbles entirely rounded; others have their edges merely blunted. Some appear so tender and so little capable of resistance, that it is difficult to conceive how they have been transported without being entirely destroyed. The prevailing material of these masses of gravel, is the pale blue or light grey variety of flint, with numerous traces of the alcyonium or other similar bodies in its substance.

According to my observation, this variety of chalk flint is not so common in the southern parts of this great chain, whilst in its continuation through Lincolnshire and Yorkshire, scarce any other is to be met with. The two principal deposits of the gravel of which I have been speaking are to be seen on the summit of Gogmagog hills, and on Harston hill about five miles to the south of the former. The height of these hills may be estimated at 800 feet above the river at Cambridge.

Harston hill has been examined by Mr. Warburton, Secretary of the Geological Society, who has obligingly communicated the