THE CHALK FOEMATION. 203 



examined in the laboratory, and we now turn, as all 

 |>-eoiogists worthy of the name should, to its occurrence 

 in the field. If we look at one of the tall chalk cliffs of 

 our southern coasts, as in the ueiij;-hbourhood of Dover, we 

 shall be first of all struck with the extrerrie homot;-eneity 

 and purity of the whole formation from top to bottom, 

 through a thickness which in this neighbourhood is close 

 upon 1000 feet, and in Norfolk is more than 1100 feet. 

 This similarity of composition throughout such a vast 

 tluekness is totally rmlike what we are accustomed to 

 Observe in other rock-cliffs ^although there is some 

 approach to it in the l>lue mountain limestone of Derby- 

 shire), where we generally find alternating bands composed 

 of rocks differing both in colour and structure from one 

 another, and we are thereby led at once to conclude that 

 there must be something- very jjeculiar connected with the 

 deposition of the chalk. How was it that in the old sea 

 there were not only no currents bringing loads of sand or 

 clay to alternate with the pure white limestone, but, above 

 all, that there was not a tinge of coloviring matter to stain 

 the virgin purity of the newly-forming chalk during those 

 ages and ages of time, while drifted logs and fruits occur 

 but rarely ': 



A closer inspection of a large thickness of chalk will, 

 however, reveal the fact that there is not a complete 

 similarity in the natui'e of the rock throughout the entire 

 formati(ju. Thus, whereas in jjlaces wiiere nearly the 

 whole formation is displayed we find throughout the 

 uppjermost 400 feet layers and nodules of flint are thickly 

 distributed throughout the mass, generally forming more 

 or less well-marked lines which indicate the original 

 planes of the deposition of the rock, as we pass to a lower 

 level the proportion of these flints becomes gradually less, 

 till, after we have passed downwards through some 130 

 feet, they finally disappear, and are wanting throughout 

 the whole of the lower part of the series. Moreover, in 

 this lower chalk, cir chalk without flints, we shall find, as 

 we pass downwards, a gradual tendency to lose the pure 

 white colour (A the u]>per chalk, and to assume a buff or 

 greyish tint, while in the very lowest beds we shall not fail 



