Prof. Harhness—On the Middle Pleistocene Deposits. 549 



clays of Norfolk are succeeded by the " middle sands and gravels," 

 which frequently attain a thickness of from 50 to 60 feet. These 

 middle sands and gravels have yielded twenty- three species of shells. 

 Amongst them is Pedunculus glycimeris, a form which Mr. Harmer 

 says dies out in the newer part of tlie Eed Crag, which is 

 exceedingly rare in the Norwich Crags, hut which is abundant in 

 the middle sands and gravels. Fectunculus (jJycimeris, in a fragmentary 

 state, is one of the most abundant of the bivalves at Pulregan, in the 

 " Manure gravels" of the Co. of Wexford. Mr. Harmer also states 

 that Ostrea edulis occurs in the "middle sands and gravels." This 

 form disappears from the newer Crag beds, is not known to live 

 within the Arctic circle; and the character of the fauna of the 

 " middle sands and gravels" is decidedly less Arctic than that of the 

 lower Boulder-clays. 



The middle sands and gravels of Norfolk, like the shell-bearing 

 beds in some portions of Ireland, are succeeded by an upper 

 Boulder-clay, which possesses features showing that it originated, 

 like the lower Boulder-clay, from Arctic conditions. 



Judging from the fauna afforded by the three deposits, the lower 

 Boulder-clay, the middle sands and gravels, or their representatives 

 elsewhere in Britain or in Ireland, and the upper Boulder-clay, we 

 arrive at the conclusion, that while, on the whole, there are distinct 

 indications of the prevalence of Arctic conditions, there was, during 

 the deposition of the middle portion of the Pleistocene deposits, a 

 less rigorous climate. 



There is another interesting circumstance in connection with the 

 middle Pleistocene strata. This is the almost constant presence of 

 chalk flints among the gravels which belong to this series. In the 

 case of these deposits, as they are seen in the south-west of Ireland, 

 there are at present no Cretaceous rocks nearer than the Co. of 

 Antrim, which is at least 200 miles distant in a direct line from 

 some of the spots where the middle Pleistocene strata are found. A 

 transportation of flint-pebbles from Antrim would imply the agency 

 of a northern current, a circumstance somewhat hostile to the 

 evidence which the fauna of the " Manure gravels " aflbrds ; telling 

 us, as it does, of a less rigorous climate than that which marks the 

 earlier and later portions of the Pleistocene epoch. 



South-eastern currents probably have had more influence in the 

 transportation of these chalk flints, and they may perhaps have 

 been derived from some source which extended from England 

 to France before the greater portion of St. George's Channel was 

 hollowed out ; or before the Straits of Dover were formed. 



The various members of the Pleistocene series have sustained, at 

 different times, great losses from denudation. 



In many cases the lower Boulder-clay is altogether absent. This 

 is seen in some instances in Norfolk, where the middle sands and 

 gravels repose upon the Crag group. At Moel Tryfaen the shell- 

 bearing sands and gravels occur under somewhat similar' conditions, 

 as they rest upon the Cambrian rocks ; and it is also probable that 

 the inferior member of the Pleistocene series, the lower Bouldex*- 



