35^ Mr. Parkinson on the Stralciy 



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 Totternhoe in Bedfordshire, and at 

 ** numerous other places ; that brought from Ryegate and other quar- 

 " ries, of this stratum, south of London, is used as a fire-stone."* 



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 cha- 

 racters 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 per- 

 haps 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 amynonites^ the 

 spiral coil is disposed in a form rather approaching to that of the oval 

 than the circle. f 



* Report on Derbyshire, &c, p. 1 12. + Organic Remains, Vol. HI. PI. IX. fig. 6. 



