194 
ELEMENTS OF GEOLOGY. 
mer forest before mentioned. When these flints, probably 
long exposed in the atmosphere, became submerged, they 
were covered with barnacles, and the surface of the chalk 
became perforated by the Pholas crispata^ each fossil shell 
still remaining at the bottom of its cylindrical cavity, now 
filled up with loose sand from the incumbent crag. This 
species of Pholas still exists, and drills the rocks between 
high and low water on the British coast. The name of 
“Fluvio-marine ” has often been given to this formation, as 
no less than twenty species of land and fresh-water shells 
have been found in it. They are all of living species; at 
least only one univalve, lenta^h^^ any, and that a 
very doubtful, claim to be regarded as extinct. 
Of the marine shells, 124 in number, about 18 per cent, 
are extinct, according to the latest estimate given me by 
Mr. Searles Wood; but, for reasons presently to be men¬ 
tioned, this percentage must be only regarded as provision¬ 
al. It must also be borne in mind that the proportion of 
recent shells would be augmented if the uppermost beds at 
Bramerton, near Norwich, which belong to the most mod¬ 
ern or Chillesford division of the Crag, had been included, as 
they were formerly, by Mr. Woodward and myself, in the 
Norwich series. Arctic shells, which formed so large a pro¬ 
portion in the Chillesford and Aldeby beds, are more rare 
in the Norwich Crag, though many northern species—such 
as Rkynchonella psittacea^ Scalaria Groenlandica^ Astarte bo- 
iiucula Cohboldice. Tellina oUiqua. 
realis^ Panopcea Norvegica^ and others — still occur. The 
Nucula CobholdicB and Tellina obliqua^ Figs. 119 and 120, 
before mentioned, p. 194, are frequent in these beds, as are 
also Littorina littorea^ Cardimn edule^ and Turritella commu¬ 
nis^ of our seas, proving the littoral origin of the beds. 
OLDER PLIOCENE STRATA. 
Red Crag. —Among the English Pliocene beds the next in 
antiquity is the Red Crag, which often rests immediately on 
the London clay, as in the county of Essex, illustrated in the 
accompanying diagram. 
