168 OLDER PLIOCENE FORMATIONS. [Ch. XIV 



where, like the Norwich beds already described, they are called " Crag," 

 a provincial name given particularly to those masses of shelly sand which 

 have been used from very ancient times in agriculture, to fertilize soils 

 deficient in calcareous matter. The relative position of the " Eed Crag" 

 in Essex to the London clay, may be understood by reference to the ac- 

 companying diagram (fig. 148). 



Fig. 148. 

 Crag. London Clay. Chalk. 



These deposits, according to Professor E. Forbes, appear by their im- 

 bedded shells to have been formed in a sea of moderate depth, usually 

 from 15 to 25 fathoms, but in some few spots perhaps deeper. Yet they 

 cannot be called littoral, because the fauna is such as may have extended 

 40 or 50 miles from land. 



The Suffolk Crag is divisible into two masses, the upper of which has 

 been termed the Eed, and the lower the Coralline Crag.* The upper 

 deposit consists chiefly of quartzose sand, with an occasional, intermixture 

 of shells, for the most part rolled, and sometimes comminuted. In many 

 places fossils washed out of older tertiary strata, especially the London 

 Clay, are met with. The lower or coralline Crag is of very limited ex- 

 tent, ranging over an area about 20 miles in length, and 3 or 4 in breadth, 

 between the rivers Aide and Stour. It is generally calcareous and marly 

 — a mass of shells, bryozoa,f and small corals, passing occasionally into a 

 soft building-stone. At Sudbourn, near Oxford, where it assumes this 

 character, are large quarries, in which the bottom of it has not been 

 reached at the depth of 50 feet. At some places in the neighborhood, 

 the softer mass is divided by thin flags of hard limestone, and corals 

 placed in the upright position in which they grew. 



The Eed Crag is distinguished by the deep ferruginous or ochreous 

 color of its sands and fossils, the Coralline by its white color. Both for- 

 mations are of moderate thickness ; the Eed Crag rarely exceeding 40, 

 and the Coralline seldom amounting to 20 feet. But their importance is 

 not to be estimated by the density of the mass of strata or its geographical 

 extent, but by the extraordinary richness of its organic remains, belonging 



* See paper by E. Charlesworth, Esq. ; London and Ed. PhiL Mag. No. xxxviii. 

 p. 81, Aug. 1835. 



\ Ehrenberg proposed in 1S31 the term Bryozoum, or " Moss-animal," for the 

 molluscous or ascidian form of polyp, characterized by having two openings to 

 the digestive sack, as in Eschara, Flustra, Retepora, and other zoophytes popu- 

 larly included in the corals, but now classed by naturalists as mollusca. The 

 term Polyzoum, synonymous with Bryozoum, was, it seems, proposed in 1830, or 

 the year before, by Mr. J. V. Thompson, but is less generally adopted. The ani- 

 mals of the Zoantharia of Milne Edwards and Haime, or the true corals, have 

 only one opening to the stomach. 



