410 Geological Society. 



Norwich crag is the Cyrena trigonalis, found also at Southwold 

 and Crostwick. The land shells consist of Helix hispida, H. ple- 

 bium, and a species found at Southwold by Capt. Alexander, bearing 

 a strong resemblance to Helix Touronensis, so common in the faluns 

 of Touraine. All the 92 marine species, excejDt two or three, are 

 found either in the red crag or living, so that a very small number 

 were peculiar to this period. It is important to notice, that a large 

 proportion of the recent shells in the coralline crag have not been 

 met with in red or Norwich ; but this absence Mr. Lyell attributes 

 to the fragile nature of many of these shells, and in some cases to 

 their having been peculiar to deep or tranquil seas. 



In determining the above results, the utmost care was taken to 

 exclude all those shells which might have been washed out of the 

 red crag into the Norfolk, or did not live in the waters by which the 

 latter was deposited. 



Should these numerical conclusions hereafter require some mo- 

 dification, still the Norwich crag will be referable to the older 

 Pliocene period, and the red and coralline to different parts of the 

 Miocene. 



From an equally careful examination by the author, Mr. Wood, 

 and Mr. G. Sowerby, of the testacea obtained in the superficial 

 lacustrine or fluviatile deposits at Cromer and Mundesley in Norfolk, 

 Stutton, Grays, Ilford, and other places near London, it appears, 

 that the proportion of recent shells in those accumulations is still 

 greater than in the Norwich crag, exceeding 90 per cent., and, con- 

 sequently, that they must be placed among the newer Pliocene 

 strata. 



In a paper communicated to the British Association at Bristol in 

 1835, Mr. Charlesworth adopted a similar chronological arrange- 

 ment of the formations above the London clay in the eastern coun- 

 ties, placing the coralline crag at the bottom of the series, the red 

 crag next in ascending order, then the Norwich (maramaliferous) 

 crag, and, highest, the lacustrine strata. In that paper Mr. Charles- 

 worth states, that the proportion of recent to extinct species had 

 not then been determined ; and Mr. Lyell remarks, it is satisfactory 

 to find, that the palaeontological test of age, derived from the relative 

 approach to the recent Fauna, is perfectly in accordance with the 

 independent evidence drawn from superposition and the included 

 fragments of older beds. 



The memoir contains also a general comparison of the fossils of the 

 crag with those of the faluns of Touraine. "When M. Desnoyers, in 

 1825, assigned a contemporaneous origin to both these formations, 

 Mr. Lyell dissented from the conclusion, 1st. because the per-centage 

 of recent species then ascribed to the crag, and determined chiefly 

 from fossils of the Norwich beds, was greater than that of the Tou- 

 raine deposit ; and, 2ndly, because the fossils are not only almost 

 entirely of distinct species, though only 300 miles distant from each 

 other, but that the Fauna of the crag has a northern aspect, and 

 that of Touraine an almost tropical character. A recent examina- 

 tion, by Mr. S. Wood, of a series of Touraine shells procured from 



