1849.] PRESTWICH ON THE CRAG AT CHILLESFORD. 349 



nation of these clays. On Tunstall Heath these same laminated clays 

 were visible in several pits. Neither had I the time necessary for 

 a further research in this district, and I beg therefore to direct the 

 attention of other geologists to this interesting field of observation. 



I have appended a list of a few localities where some illustrative 

 sections of the several deposits are exhibited*. 



Conclusions. 



From the few facts above detailed it would at all events appear 

 that the coralline and red crags are on the hills north of Orford over- 

 laid unconformably by a deposit ten to twenty feet thick of a distinct 

 mineral and palseontological character. In Iken parish this deposit 

 clearly reposes upon the coralline crag, and at Chillesford the red 

 crag is at one point shown to occur below it, and at another to out- 

 crop at a short distance from it on a lower level. The red crag is 

 also frequently found between these two places on the slopes of the 

 hills, the summits of which are capped by the clays and sands. The 

 following section would therefore, I infer, represent the structure of 

 this part of the country. 



Fig. 3. 

 Section from Iken to Chillesford. 



Chillesford Brick- Black Walks Wood. Iken 



church. 1 kiln. 2 1 brick-kiln. 



S.W. 3 3 4 N.E. 



1. Fossiliferous clays and sands C'b and c"). The clays are usually of a light bluish grey 



colour, are micaceous, and laminated commonly with yellow and ferruginous sands. 

 Their organic remains are generally in casts, and only occasionally are the shells pre- 

 served. The sands "c" are very variable, and at their base are frequently patches of 

 gravel. 



2. Sands. 3. Red crag. 4. Coralline crag. 



The distinctive peculiarity, in the Crag district, of such beds as 

 those of Iken and Chillesford, consists in their argillaceous structure, 

 their evidently tranquil accumulation, and in the rare circumstance 

 that the organic remains are imbedded in them mostly in their normal 

 position, and in a perfect state of preservation, — conditions not com- 

 mon in the coralline crag, and extremely rare in the red crag. The 

 latter especially, as it is well known, presents every appearance of a 

 most disturbed condition of the waters in which it was accumulated ; 

 there is scarcely a tranquil spot in its whole area. In these overlying 

 beds, on the contrary, the state of quiet appears to have been most 

 perfect. This is important, as we may hope to obtain, under 

 these circumstances, clear palseontological proofs of their age. In 

 the red crag period the currents and movements of the sea were 

 apparently so strong and constant, and the extent to which the coral- 

 line crag was worn down and abraded by these causes is so evident, 

 that the determination of the organic remains proper to the red crag 

 is attended with some uncertainty, as it must at times be difficult to 



* See Note at end. 



2 B 2 



