American and English Fossil Floras. 495 



defined in England, geologists had not generally accepted the theory 

 of evolution. The lines then drawn were supposed to be definite, 

 and to mark the almost extinction of one set of organisms and the 

 appearance of entirely fresh forms of life. It is at least desirable 

 that a system of stratigraphy based on such diiferent ideas to those 

 which now obtain, should be reviewed by our present lights, and 

 tlie old correlations revised by direct comparison with the original 

 standards. 



The Neocomian of England. — The Cretaceous series commences iu 

 Western France and England with the Neocomian or Lower Green- 

 sand. The term must be regarded as merely a geological expression 

 to embrace the marine formation between the freshwater deposits 

 of the Wealden of England and Germany and the Gault, It embraces 

 in England two different formations, apparently deposited in different 

 sea-basins and with relatively little in common. The one is repi'esented 

 by the formations of Speeton and Teal by on the Yorkshire, Lincoln- 

 shire, and Norfolk coasts, and is traceable through Holland and 

 Germany to Brunswick, and appears in Heligoland. This is not 

 known to exceed 500 feet in thickness, and is composed of clays, 

 grits, sandstones, pisolitic iron, and limestone, all betraying a more 

 or less littoral origin. The southern basin exceeds 800 feet thick- 

 ness in the Isle of Wight, and is possibly thicker in Kent, and 

 is composed of clays, ferruginous sands, hard '•' rags," and a variety 

 of beds which are not persistent over any large areas and change 

 their nature frequently. The fossils from various localities differ 

 considerably from each other, especially when those of the two 

 basins are compared, but all seem to betray the not distant shore. The 

 larger bivalves are the characteristic mollusca, and are undoubtedly 

 nearly related to those of the Jurassic, e.g. Gervillia, Trigonia, 

 Perna, Sphcera, Cardium sphcBroideum, Diceras, etc. The Gasteropods 

 are more typically Cretaceous, but show no approach to Tertiary 

 forms. Plants are abundant, but consist exclusively of Ferns and 

 Gymnosperms. 



The Gaidt of England. — Next in succession to the Neocomian, 

 with which we may include the Aptian, is the Gault — a term 

 originally restricted to a tract of nearly horaogeneovis blue mud, 100 

 to 200 feet thick, in the South of England and adjacent parts of 

 France. The fauna is exceedingly rich, and the presence of several 

 deep-water forms, such as NecBva, Cadidus, Leda, etc., indicates that 

 it was deposited in a considerable depth of water. A number of 

 well-preserved plants, amber, wood, etc., seem to attest that it was 

 not wholly removed from the influence of a great river. The term 

 has been extended by Eui'opean palseontologists to other scarcely 

 synchronous, and probably more littoral deposits from Central 

 Europe. The true Gault contains ample evidence that it was 

 deposited in a steadily increasing depth of water, and it evidently 

 corresponds with the Blue mud of the " Challenger " expedition, and 

 like it passes almost insensibly into a true deep-sea deposit, re- 

 presented by the Chalk Marl. Its mollusca, like those of the 

 Neocomian, are valuable for the purpose of comparison with those of 



