2 1 8 The Founders of Geology LECT. 



of shells gradually diminished until they disappeared, the 

 waters either no longer containing them or being unable 

 to preserve them. 



The gypseous series which succeeds offered to Cuvier 

 and Brongniart an excellent example of what Werner 

 termed a " geological formation," inasmuch as it presents a 

 succession of strata very different from each other, yet 

 evidently deposited in one continuous sequence. Cuvier 

 had already startled the world by his descriptions of some 

 of the extinct quadrupeds entombed in these deposits. In 

 calling attention to the occurrence of these animals, the 

 authors refer to the occasional discovery of fresh-water 

 shells in the same strata, and to the confirmation thereby 

 afforded to the opinion of Lamanon and others, that the 

 gypsum of Montmartre and other places around Paris had 

 been deposited in fresh- water lakes. 



They saw the importance of a thin band of marl at the 

 top of the gypseous series which, in spite of its apparent 

 insignificance, they had found to be traceable for a great 

 distance. Its value arose from its marking what would 

 now be called a lithological horizon, but even more from its 

 stratigraphical interest, inasmuch as it served to separate 

 a lacustrine from a marine series. All the shells below 

 this seam were found to be fresh- water forms. Those in 

 the seam itself were species of Tellina, and all those in the 

 strata above were, like that shell, marine. The two 

 geologists, struck by the marked difference of physical 

 conditions represented by the two sections of the gypseous 

 series, had tried to separate it into two formations, but had 

 not carried out the design. 



Higher up in the series, above a group of sands and 



