194 PKOCEEDINGS OF THE GEOLOGICAL SOCIETY. 



shallow, probably from elevation of the sea-bottom, until at last 

 the Gryphcea itself disappears, the bands exhibit traces of the in- 

 fluence of currents, and become more gravelly ; lignites, indicating 

 a shallow sea, become common, form belts in the ferruginous sand, 

 and in one place a bed in the wavy blue sand, at a time when much 

 iron was deposited. The deposition of the peroxide of iron appears 

 to have been connected with the disappearance of the majority of 

 mollusca, though Trigonia, Thetis, and Venus occasionally occur 

 in considerable numbers. In the uppermost strata scarcely any 

 animal remains are found, and every thing appears to indicate a 

 shallow and barren sea, previous to a new state of things, when a 

 fresh series of clays (forming the Gault) being deposited, the 

 majority of the animal forms which characterise the clays of the 

 Lower Green Sand disappear, and are replaced by distinct species, 

 representative in time. 



§ Bearing of these Observations on the Neocomian Question. 



These statements regarding the distribution of organic remains 

 and indications of mineral conditions, presented by the Atherfield 

 section, lead to a few considerations which bear importantly on the 

 question which has been agitated respecting the separation of the 

 lower part of the Lower Green Sand as a separate bed under the 

 name of " Neocomien. n 



1st. It would appear that there is but one system of organic re- 

 mains throughout the series of beds, entitled Lower Green Sand, 

 in this locality, and that whenever similar conditions are repeated, 

 the same species reappear. 



2d. Throughout the series of beds examined, we find that when 

 a species is extinguished by a change of mineral conditions, it is 

 not replaced by a representative species. 



3d. That the influences which determine the distribution of 

 species throughout are mineral and local, and that these mineral 

 — in a great measure, chemical — conditions enable us to divide 

 the strata into groups, which groups, being from their very nature 

 local, cannot be regarded as other than artificial, and have no 

 claim to be numbered as subdivisions in time of the great series of 

 cretaceous deposits. 



A change of mineral conditions may determine the absence of 

 certain species ; but, unless when, under a repetition of similar 

 mineral conditions, such species are replaced by representative 

 species, or the general assemblage of species is replaced by repre- 

 sentative and distinct forms, the change cannot be considered as 

 indicating a great sectional division. 



It appears to us, therefore, that the evidence of the Atherfield 

 section maintains the unity of the Lower Green Sand ; and that 

 the accumulation of clays at its base can be regarded only as a 

 local phenomenon. 



