STRATIGRAPHICAL GEOLOGY. 503 



ment of facies of a geological horizon was characterised by a 

 particular kind of fauna, and that certain genera and species, 

 however frequently they might occur in some lithological 

 deposits, were entirely excluded from rock-deposits of the 

 same age, but with a different lithological constitution. 

 Gressly described in great detail the various forms of rock- 

 facies which occur in the Solothurn Jura (mud facies, coral 

 facies, sponge facies; pelagic, sub-pelagic, littoral facies, etc.), 

 named the fossil types which were characteristic of each, and 

 judging both from the mineral constitution of the rocks and 

 the fossil organisms contained in them, he drew conclusions 

 regarding the mode of origin of the respective rock-formations. 



The differences between littoral deposits, shallow-water and 

 deep-sea deposits were distinguished, and also the variations 

 exhibited by deposits accumulated in the open ocean, or in 

 partially enclosed basins. Examples were likewise given of 

 transitional or passage beds in areas connecting any two 

 characteristic facies-developments. On the whole, Gressly 

 found that the facies variations in the Solothurn Jura were 

 insignificant in the Triassic, Liassic, and older Oolite deposits, 

 .but were extremely important in the Middle and Upper 

 divisions of the Oolitic series. (Observations geohgiques sur 

 I e Jura Sokurois^ 1838-40-41.) 



By a remarkable coincidence, a French geologist arrived 

 theoretically at views closely resembling those demonstrated 

 by Gressly in the field. Constant Prevost, in 1838, contributed 

 an article to the Bulletin of the French Geological Society, 

 which had special reference to a previous memoir by 

 Prestwich. Prevost explained how in each geological epoch 

 there must be contemporaneous deposits of pelagic, littoral, 

 fluvio-marine, fresh-water, and terrestrial origin, replacing one 

 another locally. Hence the mere lithological character of a 

 rock-deposit could never determine its geological age. Prevost 

 also elucidated the correlation of the faunal types with the 

 various kinds of deposit. Calcareous deposits would, he said, 

 always contain other forms of organic life than arenaceous or 

 argillaceous deposits ; on the other hand, deposits of the same 

 lithological character, although of different geological age, 

 might contain very similar fossil types. As an example of the 

 varying constitution of contemporaneous deposits Prevost 

 cited the coarse limestone, the siliceous limestone, and the 

 gypsum of the Paris basin ; while he illustrated the occurrence 



