162 Mr. Webster on the Strata lying over the Chalk, 



it is an important observation that there is no correspondence be- 

 tween the irregular form of the bottom and that of the present sur- 

 face of the country. 



Although the number of distinct beds or layers in this basin is 

 very considerable, yet the authors of the memoir have reduced them 

 to eleven principal classes. 



1. Chalk. 



2. Plastic clay. 



S. Coarse limestone and sandstone. 



4. Siliceous limestone. 



5. Gypsum and marl, containing bones of animals, forming the 



lower freshwater formation. 



6. Maries of marine origin. 



7. Sand and sandstone without shells. 



8. The superior marine sandstone. 



9. Buhr or millstone formation without shells, and argillaceous 



sand. 



10. The upper freshwater formation, comprehending marles and 



buhrs with freshwater shells. 



11. Alluvium or earth of transportation, both ancient and modern, 



analogous to our gravel, &c. comprehending rounded peb- 

 bles, pudding stones, argillaceous marles and peat moss. 



Of these the three first above the chalk are of marine origin, and 

 they cover the whole of the bottom of tlie basin. 



The gypsum and accompanying marles they imagine to have been 

 formed chiefly in fresh water, from the fossils contained in them. 



The next series of marles and sandstones containing only marine 

 shells, shows the sea to have again covered the last formed strata. 



Lastly, the upper freshwater formation demonstrates this place to 

 have been a second time converted into a lake. 



