L imestone Rocks. 5 6 1 



layers of flint, that once existed above the present sur- 

 face. The following diagram will explain this : 



FIG. 114. 



1, Chalk without flints. 2, Chalk with flints, a a, the present 

 surface of the ground marked by a dark line, b J, an old surface of 

 ground, marked by a light line. Between a a the surface is covered 

 by accumulated flints, the thickness of which is greatest where the 

 line is thickest between a' and x , above which surface a greater 

 proportion of chalk has been dissolved and disappeared. 



An irregular mixture of clay with flints, often 

 several feet thick, is also frequent on the surface of the 

 Chalk Downs on both sides of the valley of the Thames. 

 The flints, though sometimes broken, are in other respects 

 of the shape in which they were left by the dissolving 

 away of the Chalk, and the clay itself is an insoluble 

 residue, originally sparingly mingled with that lime- 

 stone. 



There is no doubt but that the plateaux of Car- 

 boniferous Limestone of the Mendip Hills, of Wales, of 

 Derbyshire, and of the north of England, have suffered 

 waste by solution, equal to that of the Chalk, only 

 from the absence of flints in these strata we have no 

 insoluble residue by which to estimate its amount. In 

 Lancashire, north of Morecambe Bay, in Westmore- 

 land, and in Yorkshire, east, north-east, and north- 

 west of Settle, the high plateaux of limestone are often 

 for miles half bare of vegetation. The surface of the 

 rock is rough and rugged from the effects of rain-water 

 and the carbonic acid it contains ; looking, on a large 

 scale, like surfaces of salt or sugar half dissolved. The 

 joints of the rock have been widened by this chemical 



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