792 HISTORICAL GEOLOGY. 



but still with wide differences between the continents as to the extent of 

 such seas. 



It is an interesting fact, bearing on the conditions under which the 

 Liassic beds were made, and the facility with Avhicli the clear open waters of 

 a fossiliferous limestone horizon may change to the confined waters of a sea 

 border, that a bed of limonite or ferruginous limestone occurs in the Lower 

 Lias northwest of Lincolnshire, England, which is 27 feet thick, and in the 

 Upper Lias, near Bath, two feet thick ; on the continent, in Ijorraine, in the 

 Upper Lias, 10 to 50 feet thick, containing Ammonites, a Gryphcea, Trigonia 

 navis, etc. ; in Auxois, France, near the base of the Lower Lias overlying a 

 bed of "lumachelle" limestone; and, as stated by C. Moore, in western 

 Australia, in the Middle Lias, a very ferruginous limestone, which on analysis 

 gave 49 to 50) per cent of metallic iron. Moore goes so far as to regard the 

 ferruginous bed of Australia as proof of Liassic age ; the associated fossils 

 are much better evidence. 



But with all the resemblance in physical conditions between Europe and 

 America, there was a remarkable contrast in the abundance of marine life in 

 the continental seas. This contrast was especially great in the Jurassic 

 period. The number of species of Jurassic Invertebrates thus far described 

 from the American rocks is less than 250 ; very few of these are Corals, 

 17 are Cephalopods, 5 Echinoderms, 17 Gastropods, 113 Lamellibranchs 

 (Whitfield). In the Jurassic of Great Britain alone the number of known 

 marine species, as stated by Etheridge {Geol., 1885), is over 3900 ; those of 

 Corals 236, Echinoderms 208, Ammonites 417, Belemnites 112, Gastropods 

 988, and Lamellibranchs 1319. ]\Iore study may quadruple the number of 

 American species ; but this will little diminish the contrast. 



As indications of the climate of the Triassic and Jurassic periods, there 

 are these pertinent facts from the Arctic regions : th^t the species Ceratites 

 Malmgreni, Ammonites Gaytani, Nautilus Nordenski'Oldi, Halohia Lommeli, 

 H. Zitteli were living in the Spitzbergen seas during the Triassic period j 

 and Ammonites {Harpoceras) 3I'Clintocki, 3fonotis septentrionalis, and species 

 of Pleicrotomaria and Nacula, about Bathurst Island, Exmouth Island, and 

 Prince Patrick Island, probably during the Jurassic period, — species that have 

 closer relations to European than to American species (Haughton, Waagen) ^ 

 that IchtJiyosaurs were living in Triassic or Jurassic time about Exmouth 

 Island (77° 16' N., 96° W.), their remains having been found on this island 

 by the Belcher Expedition ; and that other IchtJiyosaurs existed in the Spitz- 

 bergen seas, probably during the Triassic period, remains of two having been 

 found by Nordenskiold, which have been named, by Hulke, Ichthyosaurus 

 NordensTiioldi and I. Polaris; that another Saurian — "probably a Dinosaur, 

 allied to the Anchisauridae," inhabited the region about Bathurst Island, 

 Captain Sherard Osborn having brought home a vertebra, which has been 

 made a basis of a species named by A. L. Adams Arctosaurus Osborni. 



The continent of jSTorth America, as already explained (page 47), is 

 peculiar in climatal situation. It has the Gulf Stream warm with tropical 



