INTRODUCTION. vii 



2. The age of the different portions of the Tied Crag. — The physical structure of the Red 

 Crag just discussed assists, we think, in the elimination of the age of its different portions, as 

 indicated by their organic remains. The author of the ' Crag Mollusca ' very long ago, and 

 again in 1866, 1 pointed out an affinity between the organic contents of the Walton Naze bed 

 and the Coralline Crag, which distinguished the former from the rest of the Red Crag. In 

 this he was guided both by the predominance at Walton of certain species characteristic of the 

 Coralline, and by the absence of others specially characteristic of the rest of the Red Crag. 

 Thus, it is doubtful whether there has ever occurred at Walton any of the three species of 

 Tellina, T. obliqua, T. pratenuis, and T. lata, the individuals of which make up the principal 

 part of the shells of the Butley 2 and Scrobicularia Crags, and of the Muvio-marine and 

 Chillesford beds ; nor has there occurred there the several species of the genus Leda, so 

 common in the Crag of Butley ; and particularly is there absent from Walton the shell so 

 highly characteristic of the newer part of the Red Crag, of the Fluvio-marine and Chilles- 

 ford beds, and of the Lower, Middle, and Upper Glacial formations, Nucula Cobboldia ; 

 while the dextral form of Trophon antiquus, which abounds in the rest of the Red Crag, 3 

 is unknown at Walton, the sinistral or older form 4 alone occurring there. 



In testing the age of the different portions of the Red Crag, we must reverse 

 those rules which make negative evidence of so little value, and positive evidence of so 

 much ; for it is the negative, and not the positive upon which we must rely in this case. 

 Great stress has been laid upon the leavening of the Red Crag fauna by shells derived 

 from the Coralline, but the inclination of modern research is to attach less importance to 

 Coralline Crag derivation than formerly. It is, however, Red Crag derivation that seems 

 to us to be the complicating element. It is obvious that where banks of sand full of 

 shells are swept away and reaccumulated, the dead shells of these banks must be undis- 

 tinguishably mixed up with those that have but just parted with the living animal ; so that 

 if, as probably was the case, a very material change in the denizens of the Crag sea was 

 taking place between the earliest and latest parts of the Red Crag formation, by the dis- 

 appearance of certain old forms and the incoming of some new ones, we should get no 

 clear record of it in the beds of Crag formed during this time ; because the existing fauna 

 would in these contemporary accumulations be largely leavened by heaps of semi-fossilized 

 shells of the antecedent period, derived from the destruction of the older banks, such 

 as the Walton bed. 5 



1 ' Quart. Journ. Geol. Soc.,' vol. xxii, p. 538. 



2 T. lata is rare in the newer beds of the Red Crag, but profuse in the Fluvio-marine and Chillesford 

 beds. T. obliqua is a Cor. Crag, shell. 



3 Except in that of Bentley, the Crag of which presents a nearer approach to that of Walton than any 

 other, although it has a decidedly newer facies. 



4 See the remarks as to this shell, p. 19 of the ' Supplement to the Crag Mollusca.' 



5 Collectors often clear out from their boxes,in order to make room for other specimens which they value 

 more, shells obtained at other localities. These thrown away specimens may be found by the next comer, 

 and spurious localities thus arise for shells. We anticipate that much confusion will arise from this, and 



