The plastic and London clays constitute the lower beds of the series immediately above the chalk, and are nearly co-extensive in their range. From Reading on the west, these sediments stretch eastward through the valley of the Thames along the right bank to Margate; on the left, they cover the entire district to Ipswich; and constitute a very large part of the soil of the adjacent counties from Norfolk to Hampshire, prevailing more especially through the central and eastern districts. Mr. Prestwick has recently shown, that the lower English tertiaries form several distinct subdivisions, each marked by different conditions, and these conditions indicating ancient hydrographical and palæontological changes of importance. A conglomerate bed of round flint pebbles, mixed with yellow, green, or ferruginous sands, extends almost uninterruptedly from the Isle of Wight to Woodbridge, in Suffolk; this bed underlies the London clay, intercalated betwixt it and the plastic clay, and forms a well-marked geological horizon, dividing this formation from the older Eocene deposits. It contains thirty known, and eight or ten still undescribed species of testacea, twenty of which are not found in the lower deposits, while all are nearly identical with those of the superior and London clay beds. The plastic formation thus embraces the London clay, as the chalk does both, which again in its turn is embraced by the oolites; whence the older and _inferior systems_ all widen, and extend successively as the bed of the sea was elevated; and hence the basins were gradually narrowed and contracted as they approached the last and closing ante-human epoch. A kind of convergency in all this can be distinctly traced in the superficies of the earth to the state which it has now assumed. A similar approximation in its living inhabitants, as will immediately appear, can as clearly be pointed out to its present occupants. Intelligent will and design are equally manifest in the arrangements; for, however great the amount of change, the restraining hand of foresight is visibly present in them all, and, in every successive advance to the present order of things, a purpose is discernible in making the more effectual provision for the permanent stability of the human system.