414 A. J. JUKES-BROWNE ON THE HESSLE 
volved therein were properly substantiated ; with the same proviso 
he would be right in saying* that, ‘‘if we relegate these Hessle beds 
to the Glacial group, we abandon the age of deposits as the basis of 
their nomenclature, for the conditions under which they have been 
accumulated ;” and thus, to be consistent, we should bring the Glacial 
Period down to the present time. 
But is the evidence for the synchronism of the early river-gravels 
with the Hessle gravel and sand so clear and convincing as Mr. 
Wood believes? Mr. Geikie appears to accept it without question. 
Tventure to think, on the contrary, that this identification has been 
made on very slender grounds. 
In the first place, no direct evidence has ever been obtained. All 
the localities mentioned by Mr. Wood are beyond the present known 
extension of the Hessle Clay; and therefore the best test, that of 
superposition, cannot be appled. No series of river-gravels com- 
parable with those in Cambridgeshire has yet been traced under 
the Hessle Clay. I looked carefully for such beds in the Steeping 
valley, but I found a remarkable absence of river-grayels. It is 
true that Mr. Wood has made one attempt in this direction ; he 
says t:—‘‘ The gravels of this formation, which, in the Cambridgeshire 
Fen, extend southwards to about lat. 52° 30’, are there and over the 
Fen northwards uncovered by Boulder-clay ; but in Holderness they 
are so covered.” Here he assumes that the gravels in Cambridge- 
shire and Yorkshire belong to one and the same formation; but he 
cannot point to any kind of physical connexion between the two, 
and I have published elsewhere the evidence which leads to the con- 
clusion that the March gravels are the estuarine termination of the 
oldest river-courses in Cambridgeshire t, while the Holderness beds 
are marine deposits of a littoral character: the argument therefore 
rests on an assumption, and becomes misleading ; for it assumes that 
the former beds are older than the Hessle Clay because the latter are, 
and concludes that the latter are Postglacial because the former are 
supposed to be so. 
Beyond the general] considerations already indicated, there is only 
one item of positive evidence for the alledged correlation ; and this 
is the occurrence of a particular shell, Cyrena fluminalis, in both 
sets of deposits; that is to say, Mr. 8. Wood regards the occurrence 
of this shell in isolated and widely separated deposits (some of 
marine and some of freshwater origin) as evidence of their syn- 
chronism. But it seems to me that this is to strain paleeontological 
evidence beyond reasonable limits; and I venture to protest against 
the manner in which Mr. Wood employs such testimony. His 
argument concerning the beds below the Cromer till is very similar: 
he would include in the Glacial Series all the deposits which yield 
Tellina balthica—proceeding therefore on the same principle, viz. 
that the presence of one particular shell may be taken as conclusive 
evidence of contemporaneity ; and he would separate the Norwich 
Crag from the Weybourn Sands because it does not contain the 
* Geol. Mag. dec. 2, vol. v. p. 18 + Jb. dec. 1, vol. ix. p. 177. 
+ Vide The Posttertiary Deposits of Cambridgeshire, p. 59. 
