THE 

 LONDON, EDINBURGH, and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[FOURTH SERIES.] 



MA Y 1870. 



XLV. On the present state of the question as to the Limits of the 

 Jurassic and Cretaceous Periods. By F. J. Pictet*. 



UNTIL within the last few years all geologists have accepted 

 the limits of the Cretaceous and Jurassic formations as 

 they were laid down for the deposits of England and the North 

 of France. No one ventured to suppose that things may pos- 

 sibly have gone on otherwise than in the Anglo-French basin. 

 We were accustomed to assume that the Kimmeridgian and 

 Portlandian stages everywhere formed the upper limit of the 

 Jurassic period, and the Neocomian stage (including in this the 

 Valangian) the lower boundary of the Cretaceous period, these 

 limits being at some points rendered still more distinct by the 

 existence of intermediate freshwater deposits (Wealden) . 



New discoveries and investigations have thrown some doubt 

 upon this uniformity; and the general question, originating 

 almost at the same time in Germany and in France, has acquired 

 an importance and development which seem to me to render it 

 worthy of the attention of the Society. I have therefore thought 

 that I might render a service to some of our colleagues by sum- 

 ming up as briefly as possible the numerous discussions which 

 are now scattered through several periodicals, and which it is 

 not always easy to coordinate. 



The first work known to me which directly brought in ques- 



* Being a Report made to the Session of 1869 of the Swiss Society of 

 Natural Sciences. Translated by W. S. Dallas, F.L.S., from a separate 

 impression, communicated by the author, from the Bibliotheque Universelle, 

 vol. xxx vi. pp. 224-246. 



Phil. Mag. S. 4. Vol. 39. No. 262. May 1870. Y 



