1822.] Geology of the Isle of Wight, &^c. 339 



between phenomena which otherwise must have appeared 

 entirely unconnected. As the physical characters of any strata 

 become more liable to variation, all the accompanying circum- 

 stances are of proportionally greater importance. Hence, 

 arrangements of some of om formations have been made exclu- 

 sively from zoological considerations. It must, however, be 

 obvious, that classifications of this kind would be of no use to 

 the geologist, unless accompanied with direct observations on the 

 superposition of the mineral masses in which the organic remains 

 are entombed. The existence of a given suite of fossils in any 

 rock proves nothing respecting its age, unless we can show that 

 the same suite is associated with other strata of known relations ; 

 and even then, our conclusions on the subject are only founded 

 on analogy. No one ought, therefore, to generalize on such, 

 grounds as these before his observations have been widely^ 

 extended. He may then become acquainted with so many 

 corresponding facts ; that phenomena, which at first presented 

 nothing but obscure analogies, may at last become as conclusive 

 in establishing any geological relation as the most direct evi-* 

 dence. 



The truth of this statement is so obvious that it would have 

 been unnecessary to bring it forward, had it not been overlooked 

 by some of those who object to the present arrangement of cue 

 tertiary rocks. >. 



The lowest, and at the same time the most widely extended 

 deposits in the chalk basins of London and the Isle of Wight, are 

 generally divided into the two foWow'mg formations : 



1. Sand and Plastic Clay. Composed of a great many beds 

 of sand, in some places white and pure ; in others impure and tinged ' 

 with almost every possible shade of colour. Subordinate to which 

 are beds of fine potter's clay, impure argillaceous beds sometimes 

 containing calcareous concretions, beds of rolled flints, thin beds 

 of impure wood coal, &c. The beds of sand and pebbles appear 

 to have originated in a state of things ill suited to the preserva- 

 tion of organized beings : accordingly, with some remarkable 

 exceptions, we find few of their remains in this formation. 



2. London Clay. A formation of tenacious clay, often abounding 

 in sept aria ; sometimes containing thin beds of argillaceous lime- 

 stone ; containing also, more rarely, beds of sand and calcareous 

 sandstone. Organic remains are dispersed in most extraordinary 

 profusion through almost every part of this widely extended 

 formation. 



The separation of the two formations above-mentioned is not 

 marked out by any extraordinary natural epoch ; but is merely 

 assumed as a convenient classification, founded on constant 

 geological relations ; on a decided difference in the composition 

 of the constituent beds; and a still more decided difference in 



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