216 FIELD GEOLOGY. 



rations, although it cannot be denied that the scientific 

 gain in itself possesses the greatest attraction. 



To work out a very simple problem of the kind, let 

 us take the small tract surveyed in Part I., showing 

 Reading-beds, Chalk, Upper Greensand, and Gault ; the 

 last being, within the area, the lowest formation. Ke- 

 move all the others and we shall see a tract of sea- 

 bottom, on which "stone-lilies" grow and Nucula 

 pectenata flourishes; their remains being eventually 

 entombed in the increasing sediment. But the area 

 is slowly rising, and as the water decreases in depth 

 the animals migrate seawards, and others whose habitat 

 is shallow water gradually come in. One part of the clay, 

 beyond the map, is already well above the water, and the 

 new kind of sediment, sand, thins out against the some- 

 what denuded barrier the animal remains washed from 

 out the Gault being reburied with those belonging to the 

 Upper Greensand. 



The land again goes down, and continues sinking 

 for a lengthened period, during which the Chalk, with 

 its deep-sea fauna was slowly deposited. The Chalk was 

 formed in two divisions, upper and lower, with pro- 

 bably a denudation between; the upper part had de- 

 posited w r ith it and disseminated through its mass a 

 large amount of silica ; this afterwards segregated 

 itself and closed in as ifc were upon any organic frag- 

 ments containing silica, and thus formed the flint 

 concretions. Then began a long period of denudation 

 which wasted the Chalk, and left many flints upon its 

 surface embedded in the resulting stratum of clay, 

 noted at the base of the Eeading-beds in the sections. 

 This was succeeded by the deposition of the shallow 



