viii SUPPLEMENT TO THE CRAG MOLLUSCA. 



In this way it is almost impossible to separate the Red Crag into those chronological divi- 

 sions of it that probably exist ; but there are two, besides that of Walton, which may be 

 clearly indicated. The first of these is the Red Crag of Butley, in which the northern forms 

 of Molkisca predominate, and in which Nucula Cobboldia and the species of Leda 

 become very common, and of which the whole fauna, both in individuals and in species, 

 makes a great approach to that of the Eluvio-marine Crag, offering in this respect a great 

 contrast to the Crag of Walton. This approach to the one and contrast with the other 

 would, we are convinced, be greatly enhanced could we eliminate from the evidence those 

 false witnesses, the intermixed shells derived from older banks swept away. The second 

 of these beds needs no special palaeontological test for its distinction, as it rests on the Red 

 Crag of Butley, in the section under Chillesford Church (see Section XVII). It consists of 

 Crag, gradually losing both the red colour and the oblique bedding as we ascend in the section, 

 becoming horizontal in the upper layers. This Crag is poor in species, being largely made 

 up of Tellina pratemds and T. obliqua ; but in it appears Scrobicularia plana in some 

 abundance, a shell unknown in the other parts of the Red, but occurring in the Eluvio- 

 marine Crag of Bramerton. Valves of Mya truncata also, which are unknown to the 

 Walton bed, and almost so to the Crag of the Deben region, but which become common 

 in the Butley Crag, are very abundant in these Scrobicularia beds, where exposed 

 over the Coralline Crag at Sudbourn in Section XVIII. 



Although there is doubtless an intermingling of more than one stage of the Red Crag 

 in that region which is cut by the rivers Orwell and Deben, it would be impracticable to 

 distinguish them further here ; and, accordingly, in the sections all this Crag has been 

 grouped under the same symbol as the Crag of Butley, viz. as 4", although it is probably all, 

 or most of it, older than the Butley bed ; the still older bed of Walton being distinguished 

 by the symbol 4', and the newest, or Scrobicularia Crag, by 4'". 



The Chillesford Beds, and the Correlation of the Red and Eluvio-marine 



Crags. 



Although for the most part they seem to have been swept off the Red Crag region, yet 

 we find this Crag capped in a few places by some beds that remain more complete over 

 the Fluvio-marine Crag area. These consist of a micaceous sand (5') in which occurs, 

 though not constantly, the shelly bed x. This sand passes up without break into a bed 

 of laminated micaceous clay (5"), which, in some localities, yields a few shells, or their 

 casts. This bed varies from a dark blue tenacious laminated clay, as at Aldeby and 



geologists must be on their guard against it. A specimen of Tellina obliqua obtained by II. Norton, Esq., 

 of Norwich, at Walton, probably got there in this way. In Mr. Wood's collection in the British Museum 

 is a specimen of T. prcetenuis labelled Walton, which he thinks must have originally come from some other 

 locality. 



