134 



capped with an unrolled gravel, which evidently owed its existence 

 to the quiet washing away of the drift-clay from its insoluble con- 

 tents. Above Needham Market the valley is channelled in drift- 

 clay, but between Needham and Bramford it is cut through chalk ; 

 and it should be remarked, that the line of sand-hills does not extend 

 up the valley with any great distinctness beyond the chalk. The 

 phenomena seen at Creeting are not consistent with this sand lying 

 beneath the drift- clay ; and the inference is, that it constitutes the 

 remains of an estuary deposit formed in the valley subsequent to its 

 excavation in the drift-clay. 



All the other streams west of the Gipping have chalk for their 

 floor during the middle part of their course, thus manifesting the 

 existence of a ridge of chalk running beneath the drift accumu- 

 lations nearly due west and east from Sudbury to Bramford. Dis- 

 turbances evidenced in this ridge, and perhaps due to its elevation, 

 are partaken of by the London clay and crag deposits which overlie 

 it on the east and south. 



In Norfolk the drift-clay attains a greater thickness than in Suf- 

 folk, and towards the north of the county is overlaid by a sand and 

 gravel formation which may be appropriately termed upper drift. 

 The gradual disappearance of this towards the south, together with 

 the thinning away and final extinction of the drift- clay in the same 

 direction, point to a region of greater denuding activity ; it may be 

 an interesting question whether such denudation be in any degree 

 connected with the upheaval of the before-mentioned chalk ridge, 

 or again, whether the sands of the Gipping valley bear any relation 

 to the upper drift of Norfolk. 



March 13, 1854. 



A paper was read by Prof. Challis on the Eccentricity of the Moon's 

 Orbit ; supplement to a former communication on the mean motions 

 of the Apse and Node. See the former paper, Phil. Mag. vol. vii. 

 p. 278. 



Also a paper by Mr. J. Clerk Maxwell on the Transformation of 

 Surfaces by Bending. 



The kind of transformation here considered is that in which a 

 surface changes its form without extension or contraction of any of 

 its parts. Such a process may be called bending or development. 

 The most obvious case is that in which the surface is originally a 

 plane, and becomes, by bending, one of the class called " developable 

 surfaces." Surfaces generated by straight lines, which do not ulti- 

 mately intersect, may also be bent about these straight lines as axes. 

 In this way they may be transformed into surfaces whose generating 

 lines are parallel to a given plane, just as the former class are trans- 

 formed into planes. 



In both these cases, the bending round one straight line of the 



