their matrices, which would hardly have been the case had these been highly impregnated. with silex.
The hard chalk lies immediately beneath the soft chalk. In this stratum there are no flint nodules. “ Its beds,” according to Mr. Farey, “ increase in hardness, until near the bottom where a whitish freestone is dug, at Totternboe in Bedfordshire, and at numerous other places; that brought from Ryegate and other quarries, of this stratum, south of London, is used as a fire-stone.”[1]
It has been generally supposed that these two strata of chalk are of one formation: but not only the absence of the flints, but the characters of their fossils prove them to be of distinct formations. No fossils indeed are marked by more decidedly peculiar characters than those of this stratum; since hardly a single fossil has been found in it, which has been met with in the soft chalk, or any other stratum.
It is in this chalk that the genus Ammonites, is first met with, or, in other words, it appears that the water which formed this stratum was that in which this genus last existed, no traces of it having been seen in the soft chalk or in the other superior strata. The chief, and perhaps the only circular species of this genus which has been found in this stratum, is of a large size, with nodular projections on its sides, towards the back, which is generally flat. This fossil appears to be of a different species from any of those that are found in the subjacent strata.
It is very remarkable that in this stratum, the last in which the genus ammonites is met with, so remarkable a deviation from the original form of the genus should occur, as almost to claim its being considered as the characteristic of another genus. In the fossil here referred to, which possesses all the other characters of ammonites, the spiral coil is disposed in a form rather approaching to that of the oval than the circle.[2]
