BOULDER-CLAY IN LINCOLNSHIRE. 417 
and belong to the earlier or Cyrena fluminalis part of the Postgla- 
cial Period.” Now C. fluminalis does not occur in the Nar-valley 
beds ; and the O. edulis argument is of as little value as that derived 
from the former shell in other cases. If therefore Mr. Skertchly is 
right, the Nar beds as well as the Barnwell gravel, will have to be 
dissociated from the other deposits; and it is quite possible that all 
four are of entirely different ages. 
(6) The Thames Brickearths. Since these belong to an entirely 
different valley-system from that of the fen-basin, it is difficult to 
see how any absolute correlation, such as that proposed by Mr. 
Wood, can ever be established. The same remark will apply to the 
Suffolk and Essex beds containing Cyrena fluminalis. They are 
entirely separated from all the deposits which have any relation to 
the Hessle beds ; and since I claim to have shown that the argument 
derived from the presence of the Cyrena can have very little weight, 
none but the most general considerations can be brought to bear 
upon the question. Some results, of an interesting nature and 
bearing a certain amount of probability, might, indeed, be deduced 
from a comparison of the fluviatile deposits in the valleys of the 
Cam and Thames ; but the materials for this have only recently been 
obtained. It may be pointed out that the brickearths with Cyrena 
are not the oldest deposits of the Thames valley; they belong to 
the lower terraces, which are separated from the higher and older 
gravels by as wide an interval as that which marks off the Barnwell 
beds from the older gravels in Cambridgeshire. 
Mr. James Geikie* and Mr. Skertchly 7, in discussing the age of 
the Palzolithie deposits, have put forward certain theoretical consi- 
derations in support of their view that these beds are of Preglacial 
and Interglacial age. They draw attention to the fact that the 
palolithic implements are generally associated with a mammalian 
fauna which is very different from that of the Neolithic times, and 
- suggest that the gap between these two periods was caused by the 
recurrence of glacial conditions. 
Mr. Geikie asks, “* Why are paleolithic river-gravels restricted to 
the south-east of England, while neolithic remains occur broadcast 
throughout these islands? What is the reason for the limitation 
of the southern mammalia to one small area in the south-east? and 
why should the mammoth and woolly rhinoceros occur so abun- 
dantly in the yalley-gravels of that district, while they appear so 
seldom in the valley-grayels of the north?” The answer which he 
gives to these queries is, that ‘ the palzeolithic deposits are of pre- 
glacial and interglacial age, and that none of them are Postglacial.” 
His arguments are ingenious and worthy of all consideration ; but 
other answers may be given to these questions; the length of the 
postglacial period may have been greater than he supposes; and 
the evidence, though somewhat cumulative, is not uniform or en- 
tirely convincing. I am inclined to regard even the oldest river- 
gravels in the south-east of England as truly of Postglacial date, 7. ¢. 
newer than the Hessle Clay ; but I grant they may be of any age 
* Great Ice Age, p. 531. t Geology of the ae p. 208, 
G 
