126 Proceedings of the British Association, 



which are the two small outlying masses of chalk of Madingly and 

 Barton. Between the chalk and gault is a stratum, not more than a 

 few inches thick, of upper greensand full of black nodules, containing 

 50 or 60 per cent, of phosphate of lime, and many shells. The gault 

 is about 150 feet thick, and formerly when a boring was made through 

 it, the water used to rise above the surface to a sufficient height for 

 the construction of fountains, but latterly so many borings have 

 been made that the waterhead is lowered. This supply of water is 

 afforded by a bed of sand beneath the gault, which comes to the sur- 

 face about seven miles from Cambridge, on the Ely road, and is also 

 met with, parallel to the chalk hills, for a considerable distance; 

 its outcrop may be distinguished by the churches and monastic build- 

 ings placed on it, since it affords at the same time dry building 

 ground, and a constant supply of water. In this district it contains 

 scarcely any fossils. This lower greensand, as it is called, as well as 

 the gault and chalk, inclines slightly to the east, and it is this inclina- 

 tion or dip that throws up the water in the Cambridge wells and 

 fountains by hydrostatic pressure. Below the sand commences the 

 great clay of the fen district, the upper part abounding with Ostrea 

 deltoidea and the Saurian remains of the Kimmeridge clay, but 

 passing imperceptibly into the Oxford clay, without any intervening 

 coralline formation such as separates them at Weymouth, Oxford and 

 elsewhere in the south of England. Here it is only seen at one 

 place, on the river Cam, at Upware, nine miles below Cambridge, 

 where it extends for about a mile, and appears to have formed a sort 

 of pivot on which the other rocks of the country are thrown into 

 a saddle shape. The surface is in many places covered up by masses 

 of gravel and clay belonging to a more recent period, the clay being 

 part of the great boulder formation or " till," composed of drifted 

 materials brought, as the President supposed, by a rush of water from 

 the north-west over the fen districts, and also containing blocks of 

 stone derived from a more distant source. It is unstratified and 

 occupies plateaux on the hills, the valleys having been mostly formed 

 since its deposition. The red gravel is supposed to be newer than 

 the " till," and contains abundant remains of the elephant, rhino- 

 ceros, and other extinct animals. In addition to these two superficial 

 deposits are the great modern formations of peat bog, which have 



