Ill] CALCAREOUS ROCKS. 25 



in the form of regularly bedded strata stretching over a wide 

 area ; and shales or clays may be followed through a considerable 

 extent of country. The finer material composing the clays 

 and shales has been held longer in suspension and deposited 

 in deeper water in widespread and fairly horizontal layers. 



In some districts sandstones occur in which the individual 

 grains show a well marked rounding of the angles, and in 

 which fossils are extremely rare or entirely absent. The close 

 resemblance of such deposits to modern desert sands suggests 

 a similar method of formation ; and there can be no doubt that 

 in some instances there have been preserved the wind-worn 

 desert sands of former ages. Aeolian or wind-formed accumu- 

 lations, although by no means common, are of sufficient import- 

 ance to be mentioned as illustrating a certain type of rock. 



The thick masses of limestone which form so prominent 

 a feature in parts of England and Ireland, have been formed 

 in a manner different from that to which sandstones and 

 shales owe their origin. On the floor of a clear sea, too far 

 from land to receive any water-borne sediment, there is 

 usually in process of formation a mass of calcareous material, 

 which in a later age may rise above the surface of the water 

 as chalk or limestone. Those organisms living in the sea, 

 which are enclosed either wholly or in part by calcareous 

 shells, are agents of limestone-building ; their shells constantly 

 accumulating on the floor of the sea give rise in course of 

 time to a thick mass of sediment, composed in great part of 

 carbonate of lime. Some of the shells in such a deposit may 

 retain their original form, the calcareous body may on the other 

 hand be broken up into minute fragments which are still recog- 

 nisable with the help of a microscope, or the shells and other 

 hard parts may be dissolved or disintegrated beyond recognition, 

 leaving nothing in the calcareous sediment to indicate its 

 method of formation. 



Not a few limestones consist in part of fossil corals, and 

 owe their origin to colonies of coral polyps which built up 

 reefs or banks of coral in the ancient seas. 



In the white cliffs of Dover, Flamborough Head and other 

 places, we have a somewhat different form of calcai'eous rock, 



