54 



CONDITIONS OF DEPOSITION OF STRATA. 



clay. If we further suppose such an ancient coast-line to still 

 farther recede from the district when deposits are going on, so that 

 not only the sand does not reach the area, but the distance is too 

 great for even the clay to be transported so far, then none but 

 calcareous deposits can take place, due to evaporation of water or 

 evaporation combined with the agencies of plant and animal life. 

 It does not follow that the limestones were formed in deep water, 

 it is simply necessary that the sea-bed should be free from sediment, 

 or that the sediment should accumulate so slowly that its import- 

 ance is lost in the calcareous features of the deposit. Presuming 

 that these general principles are sound, then the lower Secondary 

 strata in the South of England indicate to us a great oscillation in 

 level of the land which furnished the materials for the strata. This 

 may be perhaps best expressed in a tabular form. 



Diagram showing the Altered Position of Land in the South of England 

 relatively to the Strata during the Secondary Period. 



