268 BERTHA'S VISIT TO HER 



contain a repetition of the fossils he shewed me in 

 the lower parts of it. He frequently made me 

 observe, that these fossils are all not only very 

 widely distinguished from the families found in 

 the carboniferous and transition series, but that 

 there are also striking peculiarities in themselves 

 according to the bed which they occupy. 



We came next to the great chalk formation, 

 with its wonderful deposition of flints in parallel 

 layers ; and then to the last, or superior order, 

 consisting of gravel or sand, or of clay, which is 

 in some places four or five hundred feet thick, 

 and resting on the chalk. Its organic remains 

 are highly interesting ; but my uncle said he 

 would not perplex our memories at present, by a 

 minute examination of the specimens in his col- 

 lection ; he wished to give us general ideas, here- 

 after we may study the particulars. Before he 

 closed his drawers, he shewed us, that below this 

 upper formation all the remains of organic bodies 

 were in a petrified or mineralized state ; that is, the 

 general structure and external form of the body 

 has been preserved, but the original matter of 

 which it was composed has entirely disappeared, 

 and has been replaced by the substance of the 

 mineral in which it was imbedded. On the con- 

 trary, in the strata which cover the chalk, the 

 shells are merely preserved, and in such a state, 

 that when the clay or r sand in which they lie is 

 washed off, they might appear quite recent, if 



