The least interrupted series of consecutive documents to which we can refer in the British Islands, when we desire to connect the Pliocene with the Pleistocene periods, are found in the counties of Norfolk, Suffolk, and Essex; and I shall speak of them in this chapter, as they have a direct bearing on the relations of the human and glacial periods, which will be the subject of several of the following chapters. The fossil shells of the deposits in question clearly point to a gradual refrigeration of climate, from a temperature somewhat warmer than that now prevailing in our latitudes to one of intense cold; and the successive steps which have marked the coming on of the increasing cold are matters of no small geological interest. [20] It will be seen in the Chronological Table, that next before the Pleistocene period stands the Pliocene. The shelly and sandy beds representing these periods in Norfolk and Suffolk are termed provincially Crag, having under the name been long used in agriculture to fertilise soils deficient in calcareous matter, or to render them less stiff and impervious. In Suffolk, the older Pliocene strata called Crag are divisible into the Coralline and the Red Crags, the former being the older of the two. In Norfolk, a more modern formation, commonly termed the "Norwich," or sometimes the "mammaliferous" Crag, which is referable to the newer Pliocene period, occupies large areas. We are indebted to Mr. Searles Wood, F.G.S., for an admirable monograph on the fossil shells of these British Pliocene formations. He has not himself given us an analysis of the results of his treatise, but the following tables have been drawn up for me by Mr. S.P. Woodward, the well-known author of the "Manual of Mollusca, Recent and Fossil" (London 1851-56), in order to illustrate some of the general conclusions to which Mr. Wood's careful examination of 442 species of mollusca has led. TABLE 12/1. NUMBER OF KNOWN SPECIES OF MARINE TESTACEA IN THE THREE ENGLISH PLIOCENE DEPOSITS, CALLED THE NORWICH, THE RED, AND THE CORALLINE CRAGS. COLUMN 1: NAME. COLUMN 2: NUMBER. Brachiopoda: 6. Lamellibranchia: 206. Gasteropoda: 230. TOTAL: 442. TABLE 12/2. DISTRIBUTION OF THE ABOVE MARINE TESTACEA. COLUMN 1: NAME. COLUMN 2: NUMBER. Norwich Crag: 81. Red Crag: 225. Coralline Crag: 327. Species common to the Norwich and Red Crag (not in Coralline): 33. Species common to the Norwich and Coralline (not in Red): 4. Species common to the Red and Coralline (not in Norwich): 116. Species common to the Norwich, Red, and Coralline: 19.* (* These 19 species must be added to the numbers 33, 4, and 116 respectively, in order to obtain the full amount of common species in each of those cases.) TABLE 12/3. PROPORTION OF RECENT TO EXTINCT SPECIES. COLUMN 1: NAME. COLUMN 2: NUMBER OF RECENT. COLUMN 3: NUMBER OF EXTINCT.