190 Evolution and Distribution of Fishes 



freshwater deposits on land surfaces often alternate in 

 close succession with each other. Biologically one result 

 of this was that — as in the case of Solenhofen slates — an 

 admixture of organisms may be entombed in what appears 

 to be a continuous rock mass of 40 to 80 feet in thickness. 



Broadly it may be said that the first-deposited or Liassic 

 beds are in some areas wholly or largely freshwater — as 

 is true of the Lower Lias, — in others marine as is true often 

 of Mid-Liassic beds. The lower and middle Oolites are 

 largely marine, though in Europe the "Stonesfield Slates" 

 seem to be largely freshwater deposits. The upper or 

 "Portland Oolite," and the Purbeck that corresponds to the 

 Upper Jurassic of America, are of mixed character below 

 but largely freshwater above, while the lowest or Wealden 

 beds of the Cretaceous formation are essentially of fresh 

 water or land origin. 



But before proceeding further it should now be stated 

 that a palaeontological classification of Jurassic and Cre- 

 taceous strata has come into vogue, which while undoubtedly 

 of great service in distinguishing marine strata, has tended 

 to throw into the background a true biological estimate 

 of the land and freshwater deposits of both formations. 

 Or even through the reporting loosely of two or more 

 groups of fossils from strata that are adjacent but quite 

 distinct in origin, correspondingly loose views have origi- 

 nated as to the true interpretation biologically of the 

 fossils. So while definite species of pelecypodous, gastero- 

 podous, or cephalopodous molluscs may be helpful strati- 

 graphic guides in determining marine horizons, if these 

 organisms are mixed, in tabulations, with organisms from 

 other and it may be adjacent freshwater beds, an obscure 

 or entirely incorrect view may be got as a whole. One of 

 the early and also careful observers in this respect is P. B. 

 Brodie, whose "History of Fossil Insects in the secondary 

 rocks of England (1845)" abounds in detailed classifica- 

 tions of strata with their typical fossils. 



In trying therefore to obtain a correct picture of the 

 physico-biological aspects of the Jurassic, the writer pro- 

 poses first to refer to or directly quote, descriptions of 

 previous observers. 



