248 MESSES. E. H. EASTALL AND J. KOMANES ON [Aug. I909, 



boulders is shown by the fact that in many pits similar blocks are 

 found in undisturbed gravel. However, on account of the possi- 

 bility of artificial transport for considerable distances by land or 

 especially by water, it is unsafe to found any conclusions on 

 these pavements, and all such have been here excluded. 



The method pursued in the investigation was as follows : all the 

 specimens were carefully labelled as soon as collected, and any of 

 uncertain origin were rejected. The great majority of the specimens 

 were collected from gravel-pits in actual operation, or from the 

 surface of arable land where there is little or no likelihood of 

 transport by human agency. Besides these, there were found in 

 the Sedgwick Museum many specimens of undoubted authenticity 

 collected from the Barrington and Barnwell gravels. 



After all the material had been critically examined in this way, 

 the specimens were sorted out according to locality, and for this 

 purpose the whole area was divided into districts, of which the 

 following is a list : — 



(1) Cambridge and neighbourhood ; 



(2) Hard wick and St. Neots Eoad ; 



(3) Lords Bridge, Barton, and Comberton ; 



(4) Barrington and Foxton ; 



(5) Kingston and Toft ; 



(6) Whittlesford, Pampisford, Hinston, and Linton ; 



(7) Chesterford. 



Petrological Examination. 



In view of the detailed petrological investigation of the material, 

 thin slices for microscopic examination were prepared from the 

 most promising specimens ; all the slices described in the British 

 Association Report and in the Geological Magazine were carefully 

 re-examined in the light of further knowledge, and a large number 

 of new ones were obtained, making a total of about 150. The 

 specimens which were not sliced were compared with typical rocks 

 in the Sedgwick Museum and elsewhere, and a number which 

 were believed to be of Scottish origin were taken to Edinburgh and 

 compared with specimens in the Royal Scottish Museum. Our 

 thanks are due to Mr. R. Campbell, M.A., B.Sc, of the Edinburgh 

 University, and Mr. W. T. Gordon, M.A., B.Sc, of Emmanuel 

 College, Cambridge, for their kind assistance in identifying several 

 of the specimens. Full details of the results obtained by these 

 means are given in later sections. 



When this examination was complete, the rock-types recognized 

 with certainty were plotted by means of conventional signs on a 

 1-inch Ordnance map of the district, so that the distribution of 

 boulders of varying origin could be seen in a graphic manner. 

 The results thus obtained will be discussed in the final section of 

 this paper. 



