112 GEOLOGICAL MEMOIRS. 



the beautiful geological map of France. These geologists ob- 

 served in the South of France beds of considerable thickness, 

 in which they have professed to find tertiary fossils mingled 

 with cretaceous species, and this supposed mixture occurs, not 

 between the newest cretaceous bed and the oldest tertiaries, 

 but between these last and comparatively ancient beds of the 

 cretaceous group. Being in possession of a very considerable 

 number of facts, which satisfied me that there were no species 

 common to the chalk and the tertiary strata, and knowing also 

 the observations of Messrs. Dufresnoy and Elie de Beaumont, I 

 waited till further investigations should throw some light on 

 the question. I further objected that it was not in the Py- 

 renees, where the upper member of the cretaceous group is 

 wanting, that the problem could be solved, but rather in those 

 districts where the chalk, in its most complete form, is in imme- 

 diate contact with the most ancient tertiary beds. Now it is at 

 Maestricht that this sequence is found, and it has long been affirmed 

 positively that in this well-known locality no species occurs 

 common to the newest cretaceous rocks and the oldest tertiaries 

 of the period of the Paris Basin. If the mixture of species is 

 not to be found there, where the nearest approach is made to a 

 perfect sequence, then, a fortiori, we can still less expect to find it 

 in a district where the chronological order of the formations is 

 interrupted by the absence of the upper cretaceous beds of 

 Maestricht. 



Till lately, however, the question has been left undetermined 

 in geology, and it was necessary to examine afresh the form- 

 ations in which the mixture of species had been observed. Our 

 colleague, M. Leymerie, has already given to the Society the 

 result of his conscientious and laborious researches : he has 

 submitted for your examination collections of fossils richer than 

 any hitherto known, and among all the species he has collected, he 

 has not found one common to the tertiaries and the cretaceous 

 rocks. Mr. Pratt, a careful observer, who is known to you by 

 his papers on geological subjects, and who is fully aware of the 

 importance of this matter, not being satisfied with the result of a 

 first journey to Biaritz, has undertaken another this year, and it 

 is the result of this second excursion that our excellent colleague 

 has charged me to communicate to the Society. Before his 

 departure, Mr. Pratt was impressed with the necessity of observing 

 with the greatest attention the boundary line between the ter- 

 tiary beds and the chalk : he knew also that it would not be 

 sufficient to collect the fossils at the foot of the escarpments, 

 but that he must take them from the beds themselves, and that he 

 must keep his collections distinct. The groups of fossils procured 

 by Mr. Pratt under these circumstances seem to me to possess 

 great importance, and I have examined them with the most careful 

 attention : the result of this examination has been, first, that the 

 whole of the nummulitic system belongs to the tertiary series, 

 which confirms the observations of M. Leymerie in the Corbieres 



