I have now to describe the most remarkable and best characterized of all the strata in that hill in the Isle of Wight called Headen, which has so frequently come under our examination.
Here, immediately over the last-mentioned formation, there is a thin bed of sand of 6 inches, upon which rests immediately a very extensive calcareous stratum 55 feet in thickness, every part of which contains freshwater shells in great abundance, without any admixture whatever of marine exuviæ.
The substance of which this stratum consists is of various character, although it cannot be described as being subdivided into smaller beds. A great part is composed of a yellowish white marl, sufficiently indurated to remain in blocks when fallen down, but extremely friable, and which, like other marls, will not endure the frost. In this, and disposed without any regularity, are hard masses of a rock which appears to contain a greater proportion of calcareous matter; and to be in about an equal quantity with the marl. This stone is very durable, and is employed as a building material. Between these two extremes there are many parts of intermediate degrees of hardness and durability.
Many of the shells which are found imbedded in this stratum are quite entire, and these are mixed with numerous fragments of the same species. They consist, like the lower freshwater formation, of several kinds of lymnei, helices, and planorbes, and from the perfect state of preservation in which they are found, must evidently have lived in the very spots where they now are, the shells of these animals being so friable that they could not have