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a deep ocean in a slowly subsiding area, — a condition of things, 

 probably analogous to the coral sea within 30° of each side 

 of the equator in our day. The Jurassic waters were then 

 studded with coral reefs, extending over an area equal to a 

 great portion of modern Europe, as shown by the geographical 

 distribution of the coralline formations, which stretch through 

 England in a diagonal line from Yorkshire to Dorsetshire; 

 thi-ough France, from the coast of Normandy to the shores of 

 the Mediteri'anean, forming besides a chain extending obliquely 

 thi'ough its central portion from the department of the Ardennes 

 in the north, to Charente Inferieure in the south, including 

 Savoy, the Hautes-Alpes, and Basses-Alpes ; the Jura of the 

 Haute-Saone, and the Jui'a franc- comtois, and the Swiss Jura 

 chain throughout its entire length ; from Schaffhausen on 

 the Ehine, to Coburg in Saxony, and along the range of the 

 Swabian Alps, and the Eranconian Jura. The corallian, was, 

 therefore, a widely extended formation, and appears to have 

 been formed xmder conditions similar both in their physical 

 and biological relations. 



The Ammonites that have hitherto helped us to determine 

 the limits of the Jui-assic fonnations are unfortunately rare in 

 the corallian strata, and we must seek among the leading fossils 

 of these beds for other genera, whereby to ascertain their 

 correlations; fortunately we find excellent substitutes in the 

 Corals, EcMnidce, and certain genera of Mollusca so abundant in 

 the different stages thereof. Examined by this test the corallian 

 terrains in France, Germany, and Switzerland admit of a 

 division into three zones, which, in descending order, may 

 be thus defined : — The upper zone consists of fine white earthy 

 or siliceous limestones like chalk, with numerous species of 

 Nerincea, and Diceras arietina, which characterize it. The second 

 zone is remarkable for the large number of corals it contains, 

 and which form a Madreporic Limestone; in fact, the fossil 

 reefs of a coral sea. The third zone contains many Echinoderms, 

 among which Cidaris florigemma and Glypticus hieroglyphicus are 

 conspicuous; the spines of Cidaris florigemma alone forming an 

 excellent leading fossil when the test is absent. 



