Ch. Xni.] FOSSILS OF THE RED CRAG. 201 



The relative position of the Red Crag in Essex and the subjacent 

 London clay and chalk has been already pointed out (fig. 144). 

 Whenever the two divisions are met with in the same district, the 

 Red Crag lies uppermost ; and in some cases, as in the section repre- 

 sented in fig. 149, which I had an opportunity of seeing exposed to 



Fig. 149. 



Sutton 



Section near Ipswich, in Suffolk. 

 a. Eed Crag. b. Coralline Crag. c. London Clay. 



view in 1839, it is clear that the older or Coralline mass b had suffered 

 denudation before the newer formation a was thrown down upon it. 

 At D there is not only a distinct cliff, 8 or 10 feet high, of Coralline 

 Crag, running in a direction N.E. and S.W., against which the Red 

 Crag abuts with its horizontal layers ; but this cliff occasionally over- 

 hangs. The rock composing it is drilled everywhere by Pholades, 

 the holes which they perforated having been afterwards filled with 

 sand and covered over when the newer beds were thrown down. As 

 the older formation is shown by its fossils to have accumulated in a 

 deeper sea (15, and sometimes 25, fathoms deep or more), there must 

 no doubt have been an upheaval of the sea-bottom before the cliff 

 here alluded to was shaped out. We may also conclude that so great 

 an amount of denudation could scarcely take place, in such' incoherent 

 materials, without many of the fossils of the inferior beds becoming 

 mixed up with the overlying crag, so that considerable difficulty must 

 be occasionally experienced by the paleontologists in deciding which 

 species belong severally to each group. 



The Red Crag being formed in a shallower sea, often resembles in 

 structure a shifting sandbank, its layers being inclined diagonally, and 

 the planes of stratification being sometimes directed in the same 

 quarry to the four cardinal points of the compass, as at Butley. 

 That in this and many other localities, such a structure is not decep- 

 tive or due to any subsequent concretionary rearrangement of parti- 

 cles, or to mere lines of color, is proved by each bed being made up 

 of flat pieces of shell which lie parallel to the planes of the smaller 

 strata. 



Some fossils which are very abundant in the Red Crag, have never 

 been found in the white or coralline division; as, for example, the 

 Fusus contrarius (fig. 150), and several species of Murex and Bucci- 

 num (or Nassa) (see figs. 151, 152), which two genera seem wanting 

 in the Lower Crag. 



Many of these shells are found in a good state of preservation in 

 the cliffs of Walton-on-Naze, in Essex ; at Felixstow the cliffs afford 

 fewer shells, and most of them are fragmentary. 



