RED CRAG. 
77 
species are few. In places, especially away from the centre of 
the district, the heels are almost shell-less, or the shells occur in 
patches. This may be due either to the original absence of shells, 
or to their removal by percolation of rain-water, for where the beds 
are consolidated by iron-oxide casts are sometimes met with. 
The author alludes to the difficulty of marking the line between 
the two divisions (the upper of which he once called the unpro¬ 
ductive sands,” from the absence of fossils, which, however, have 
since been found in places), and notices sections showing a line of 
erosion between them. The upper division is the Chillesford 
Sands, succeeded by the Chillesford Clay, which is also classed 
with the Red Crag. 
Details of many sections are given in the same paper, with lists 
of the fossils from some. The fossils are of two kinds, those proper 
to the formation and those derived from other formations. Of 
these latter, which are important, some may have been doubly 
derived, from the Coralline Crag, which got them from older beds. 
With regard to the Mollusca there is a difference of opinion as to 
which are derived species; but the author is inclined to think 
that the greater number of the species found in the Suffolk Crag 
but not in the Norfolk Crag have been derived from the Coralline 
Crag. 
The lamination that Wood referred to beach-action was regarded 
by Prof. Prestwich as having been produced by the shifting of 
shoals and sand-banks at the bottom of the sea, which was shallow 
and studded with reefs of Coralline Crag; whilst in winter the 
distant Chalk and Tertiary shores were fringed with ice, which 
floated off large flints and deposited them in the Red Crag. The 
constant shifting of materials would result in the heavier parts, 
as bones, pebbles, and phosphatic nodules, being left behind and 
thus tending to accumulate in the basement-beds. The conse¬ 
quence of the reconstruction is that the shells are much broken 
up. The currents seem to have flowed from a direction varying 
between east and south-east, and the Red Crag sea seems to have 
been open to the north, but probably closed on the south. There 
is no distinction in the fossils from top to bottom of the lower 
division, after the deposit of which there was a slight subsidence. 
From the lower part of the upper sands having been formed from 
the lower Red Crag there is a similarity in mineral composition ; 
but as the sea-bed became more depressed the sands became finer, 
until they passed up into the micaceous sands and clays of the 
Chillesford Beds. 
In the third part of this set of papers’^ Prof. Prestwich referred 
chiefly to the Norwich Crag area, and concluded that this was 
divided fi om the more open sea of the Red Crag by a barrier of 
Coralline Crag, streams flowing into the northern area, and 
carrying in land and freshwater shells. A list of the Mollusca 
of the Red and Norwich Crags, to a great extent on the authority 
of Dr. J. G. Jeffreys, is given, 
* Quart. Journ. Geol. Soc., vol. xxvii. pp. 452-496. 
