STRATA IN THE NEIGHBOURHOOD OF LONDON. 121 



colating fluid, by the crystallization of which these bodies 

 liavebeen formed, and are now augmenting. 



The Obei-stein nodules of agate appear to have been Oberstein no- 

 formed under somewhat eimiior tircumstances ; sinceitisin "^^*^ agate, 

 general evident from their external surfaces, that they also 

 have bad very little adherence to 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. Hardclialk 

 In this stratum there are no flmt nodules. «* Its beds," '*''*'^'^''^^'"*- 

 according to Mr. Farey, " increase in hardness, until near 

 *t.the bottom, where a whitish freestone is dug, at Tottern- 

 ** hoe in Bedfordshire, and at numerous other places; that 

 •*. brought from Kyegate and other quarries of this stratum, 

 *f south of London, is used as a fire-stone*," 



It has been generally supposed, that these two strata of A differenf 

 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 its fossils 

 more decidedly peculiar characters than those of this stra- P^*^"^'^^* 

 turn ; since hardly a single fossil has been found in it, 

 which has been met with in the s^ft chalk, or any other 

 stratum. 



It is in this chalk, that the genus ammonites is first met Ammonites 

 with ; or, in other words, it appears, that the water, which first occur in it* 

 formed this stratum, was that in which this genus last ex- 

 isted, 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, toward the back, which is generally flat. This One species 

 fossil appears to be of a different species from any of those, ^°JJ"^ '""** 

 that are found in the subjacent strata. 



It is very remarkable, that in this stratum, the last in One-withal- 

 which the genus ammonites is met with, so remarkable a ™°** ^ generic 

 deviation from the original form of the genus should occur, ' ^^ ^'^** 

 as almost to claim its being considered as the characteristic 

 pf another genus. In the fossil here referred to, which pos- 



♦ Report on Derbyshire, S(C^ p. 11?. 



