260 MESSES. B. H. EASTALL AND J. E0MA.NES ON [Aug. I9OQ, 



List op Ehomb-Porphyries. 



Grantchester. Barrington Hill, Linton. 



Chesterton Railway-Bridge. Whittlesford. 



Traveller's Best, Huntingdon Boad Half a mile north-west of Pampisford 



[2]. Station [2]. 



Vandlebury, Gog-Magog Hills [2]. Chesterford. 



St. Neots Boad, 5 miles from Impington. 



Cambridge. Bartlow. 



Comberton. , Haverhill. 

 One mile north of Barton Church. St. Ives [2]. 



Barrington [2]. ' Hitchin. 



Toft. ; Bedford [6]. 



Hinxton. ; Hunstanton [15]. 



Note. — Numerous rhomb-porphyries have been observed as paving-stones 

 in Cambridge ; but these are excluded, as their origin is uncertain. 



VII. Sttmmaey and Conclusions. 



As a result of the detailed and systematic study of the 

 far-travelled constituents of the Cambridge Drift summarized in 

 the preceding sections, certain general conclusions may now be 

 formulated. In the first place, it is obvious that material has 

 been brought together from widely different sources. It is 

 unfortunate that the characters of the Glacial deposits of Cam- 

 bridgeshire and their derivatives are such that it is impossible 

 to determine with certainty whether the arrival of all tbese foreign 

 rocks was simultaneous, or whether they were brought by successive 

 ice-streams at different times. It is generally stated that in East 

 Anglia the far-travelled rocks occur only in the lowest division of 

 the Glacial Series, which is also said to be absent from Cambridge- 

 shire. The occurrence, however, of foreign rocks in great abundance 

 throughout the county, and as far west as Bedford, indicates that 

 this division was originally deposited, but subsequently ploughed up 

 by a later ice-sheet, and incorporated in the Chalky Boulder-Clay. 



From our mapping of the occurrence of different rock-types there 

 is an indication of a partial and local distribution. One of the 

 most important pieces of evidence bearing on this is the fact that 

 Bed Chalk and bored Gryphseas occur chiefly to the east and south 

 of Cambridge, becoming much rarer to the west. On the other 

 hand, Jurassic limestones derived from the Great Oolite and Inferior 

 Oolite are very abundant in the west, about Bourn and Old North 

 Boad. Although Chalk is abundant everywhere in the Boulder-Clay, 

 it is noticeable that many of the more markedly rounded and striated 

 pebbles agree better with the hard Chalk of Lincolnshire and 

 Yorkshire than with the soft Chalk of the district. With these 

 are associated many large grey tabular flints, which probably came 

 from Lincolnshire : these have a very wide distribution. 



Turning now to the igneous rocks, we find that the highly 

 characteristic and easily determinable types of the Christiania 

 petrographical province are met with over the whole district, and 

 occur in considerable abundance at Bedford. The distribution of 



