90 Mr. Charles worth on the Crag-formation 



some places deposits would be forming, which might after- 

 wards, from an alteration in the force or direction of currents, 

 have materials of a very different nature carried down upon 

 them. On a subsequent examination of these we should not 

 only find alterations in their character and stratification, but 

 probably corresponding variations in the accompanying shells. 

 Upon this principle we could imagine that a change might be 

 exhibited in the character of a deposit, as extensive and as uni- 

 form in its nature as that presented by the tertiary strata now 

 under consideration. There are, however, some circumstances, 

 the occurrence of which would prevent our adopting this 

 theory, should it be suggested, as explanatory of the phe- 

 nomena observed in the instance before us. 



The red crag affords decisive evidence of having been a 

 gradual deposit formed by successive accumulations of marine 

 exuviae, which were not brought from a distant part by the 

 operation of a powerful current, but belonged to the natural 

 inhabitants of those localities, which, owing to the subsequent 

 retreat of the ocean, are now rendered accessible. Among the 

 most abundant of these reliquiae were numerous species of the 

 genera Murex and Buccinum, of which no traces are disco- 

 verable in the coralline crag, although its sands were admi- 

 rably calculated for the preservation of such Testacea as ex- 

 isted at the time of its formation*/ 



If we refer to the localities in which the inferior beds have 

 been exposed, we find one situate in the most central part of 

 the crag, while the others form the eastern and western boun- 

 daries. On examining these spots, there is nothing which 

 would lead us to imagine that the lower stratum is a mere 

 local deposit; on the contrary, there is every indication of its 

 extending beneath the superincumbent beds, but owing to 

 their general horizontal position, and the slight depth to which 

 artificial excavations are usually carried, there will probably 

 be considerable difficulty in ascertaining the precise limits of 

 the district it occupies. 



The hundred and fifty species common to the red and co- 

 ralline crag may, perhaps, be brought forward as a conclu- 

 sive proof of their intimate relation to each other : one con- 

 sideration, however, must not be lost sight of, namely, whether 

 some of the shells now occurring in the red crag may not ori- 

 ginally have belonged to the coralline. In some parts of Nor- 



* It should be remembered that the Murices and Buccina are by no 

 means the only absentees in the coralline crag. The number of species 

 deficient is altogether eighty, including the Tellina:, Cypr<ece, &c, while at 

 the same time we find an addition of more than 200 new species. 



