Vol. 65.] THE BOULDERS OF THE CAMBRIDGE DRIFT. 261 



this family can be seen from the list of localities at which rhomb- 

 porphyry has been found (see p. 260). Porphyry an, which is also 

 probably of Scandinavian origin, has an equally wide distribution : 

 it is abundant at Bedford, and has been recorded even from Penny 

 Stratford. Prom this it is clear that at one period the whole district 

 was within reach of Scandinavian ice. 



The presence of a considerable number of boulders derived from 

 Aberdeenshire and from the Old Red Sandstone conglomerate of 

 Porfarshire presents certain difficulties, as it is inconsistent with 

 the accepted ideas as to the general direction of flow of the 

 ice-streams on the east coast of Scotland. The evidence of striae, 

 etc., there shows that the ice from the Central Valley and the 

 South-Past Highlands spread out in a fan, part moving north- 

 eastwards, parallel to the coast, and part towards the south-east. 

 This, however, probably applies only to the ice in the immediate 

 neighbourhood of the coast ; while farther out to sea this stream must 

 have come into contact with the Scandinavian ice, and parts would 

 inevitably be carried southwards. The south-easterly direction of 

 the southern half of the fan affords a ready explanation of the 

 occurrence of boulders from the Central Valley of Scotland and the 

 Cheviots, and such rocks are common in all the drifts of eastern 

 England. Included in our collection are a great number of basalts 

 and dolerites, which we have been unable to identify in detail, but 

 which might well come from the North of England and Central 

 Scotland. 



Although the occurrence of Lake District rocks in Cambridge- 

 shire has not hitherto been recorded, there is no difficulty in 

 explaining their presence : one or more streams of ice from the 

 Lake District are known to have crossed the Pennine Chain to the 

 North Sea, and boulders of Shap granite and other unmistakable 

 types are exceedingly common on the Yorkshire coast. Their great 

 rarity in the drifts of Cambridgeshire is rather a matter of surprise 

 than otherwise. 



Among the metamorphic rocks are many gneisses and schists, but 

 from - these it is next to impossible to draw any conclusions, as 

 most of them could be matched either from the Highlands or from 

 Scandinavia. It is notable that there seems to be a tendency for 

 the gneisses to occur towards the east and the schists towards the 

 west. It is a fact of some significance, as before pointed out, that 

 the majority of the granitic and syenitic gneisses show a distinctly 

 alkaline character, and this is in favour of a Scandinavian origin. 



Of the two hypotheses advanced to explain the distribution of 

 the Glacial Drifts over the eastern part of England we prefer 

 that of land-ice, and consistently with this have arrived at the 

 following conclusions : — Pirst, a great ice-sheet advanced from the 

 North Sea and distributed Scandinavian boulders over the whole 

 district : this formed the Cromer Till and Contorted Drift of 

 Norfolk. The direction of movement of this ice-sheet is unknown, 

 but it presumably came from the north-east. At a later stage, ice 

 of British origin moving parallel to the coast became relatively 



Q. J. G. S. No. 259. t 



