192 GEOLOGY OF THE ISLE OF WIGHT. 



Summary.— The character of the cretaceous 

 system, as shown by these investigations, is that of 

 an ocean-bed formed in a vast basin by successive 

 accumulations of sedimentary detritus, transported 

 by currents, and thrown down in the tranquil 

 depths of the sea ; arenaceous and argillaceous 

 deposits prevailing in the lower, and cretaceous in 

 the upper division of the series ; periodical intru- 

 sions of heated fluids charged with silex having 

 taken place at uncertain intervals. The fossils 

 prove that the ocean swarmed with innumerable 

 beings of the usual orders of vertebrate and inver- 

 tebrate marine organisms, belonging for the most 

 part to species and genera now unknown ; and in 

 the chalk are seen for the last time that numerous 

 tribe of cephalopoda, the Ammonites, of which 

 not a single species is known either in the tertiary 

 strata, or in any more recent deposits : so far as 

 our knowledge at present extends, with the chalk 

 the whole race of Ammonites disappeared. 



With regard to the vegetable kingdom of the 

 cretaceous period, the presence of numerous fuci 

 attests the nature of the marine flora; and the 

 fragments of drifted coniferous wood, fir-cones, 

 stems and leaves, which are found in the flint and 

 chalk in some localities, prove that the dry land 

 was clothed with pine-forests, and cycadeous 



