2 1 6 The Founders of Geology LECT. 



to the white calcareous formation underneath it. It showed 

 no passage into that formation, from which, on the contrary, 

 it was always abruptly marked off, and it yielded no 

 organic remains. The two geologists accordingly drew the 

 sound inference that the clay and the chalk must have 

 been laid down under very different conditions of water, 

 and they believed that the animals which lived in the 

 first period did not exist in the second. They likewise 

 concluded that the abrupt line of junction between the 

 two formations might indicate a long interval of time, and 

 they inferred, from the occurrence of an occasional breccia 

 of chalk fragments at the base of the clay, that the chalk 

 was already solid when the clay was deposited. 



The next formation in ascending order was one of sand 

 and the Calcaire Grossier. It was shown to consist of a 

 number of bands or alternations of limestone and marl ; 

 following each other always in the same order, and trace- 

 able as far as the two observers had followed them. Some 

 of them might diminish or disappear, but what were below 

 in one district were never found above in another. " This 

 constancy in the order of superposition of the thinnest 

 strata," the writers remark, " for a distance of at least 12 

 myriametres (75 English miles), is in our opinion one of 

 the most remarkable facts which we have met with in the 

 course of our researches. It should lead to results for the arts 

 and for geology all the more interesting that they are sure." 



One of the most significant parts of the essay is the 

 account it gives of the method adopted by the explorers to 

 identify the various strata from district to district. They 

 had grasped the true principle of stratigraphy, and applied 

 it with signal success. The passage deserves to be quoted 



