33C ^44M^f' Sedgwick on ih [May, 



IV. Green Sand and Chalk Formation. 



The great geological phenomena presented by these two for- 

 mationa have been admirabl}^ detailed by Mr. Webster. Their 

 fossil history has already received considerable illustration : much, 

 however, remains to be done. The zoophytes (especially of the 

 green sand) would afford excellent materials in the hands of a 

 good naturalist, as the species are very numerous, and the 

 greater part of them remain undescribed. 



The beds of green sand in Freshwater Bay are in a state of 

 imperfect aggregation which favours the extraction of the organic 

 remains. 1 ne corresponding beds which appear near the eastern 

 extremity of the island are in a state of much greater induration. 

 They are there seen in contact with a variety of indurated marl 

 which forms the basis of the chalk deposit. The line of junction 

 is ill defined, as the green sand appears to pass into the beds 

 which repose upon it, by gradations which are almost insensible. 

 These ambiguous appearances are limited to the extent of a feW 

 feet, and can throw no difficulty in the way of a proper classifi- 

 cation of the two formations. In some other parts of England, 

 the case is far different. THe argillaceous matter, which always 

 forms a constituent of the lower chalk, prevails to such a degree 

 that the beds gradually pass into a tenacious clay. On the other 

 hand, the lower portions of this argillaceous marl become mixed 

 with sand and sandstone, which seem to link them to the greeti 

 sand formation. Hence arises an ambiguity. 



The beds of argillaceous marl may be considered as forming a 

 portion either of the green sand, or of the chalk series. Mr. 

 Greenough, in his Geological Map, has chosen the former alter- 

 native. As far as my own observations go, the choice has, I 

 think, been unfortunate. Perhaps the proper mode of avoiding 

 all ambiguity would be, to give an appropriate name to all those 

 varieties of chalk marl which are in the state of a tenacious clay, 

 and to represent them by a peculiar colour. The strata in the 

 neighbourhood of Cambridge afford an example of the arrange- 

 ment we have been describing. The order of superposition is as 

 follows : 



(1.) Chalk with flints, forming an escarpment which ranges 

 about 10 miles to the east of the town. 



(2.) Indurated chalk marl, the upper portion approaching the 

 true character of chalk, the lower portion becoming gradually so 

 argillaceous, that it at length loses the appearance of a cretaceous 

 rock. 



(3.) Tenacious bluish clay (gait) separated from the preceding 

 by a very thin bed, which is mixed with green sand, and contains 

 a great many fossils. No denudation immediately about Cam- 

 bridge shows any inferior formation ; but on the confines of 

 Bedfordshire, the gall is seen reposing on the iron sand.^ Thi»' 



• Geol. Tiaiui. v. 114. 



