I.] WEATHERING u 



can only have been wind-borne ; this rapidly increases 

 as the first vegetation checks the velocity of the 

 wind above the stones and arrests the dust, till at last 

 it reaches the surface and the grass begins to spread 

 over the stones. Exact dates are difficult to obtain, 

 but probably considerably less than a century is suffi- 

 cient to form a thin turf over a bare shingle bed. 



But the great weathering agency in temperate climates 

 is undoubtedly frost acting upon water contained within 

 the rocks and stones ; the water expands as it changes 

 into ice, and exerts an enormous pressure indeed about 

 ioo atmospheres would be required to keep water in a 

 liquid condition at iC. All rocks when freshly exposed, 

 hold, by capillary attraction, a certain amount of water 

 known as the " quarry water," which amounts in the 

 white chalk to as much as 19 per cent. A piece of such 

 chalk will be shattered into fragments by a single 

 night's frost. Even after the quarry water has been 

 dried out the most close-grained rocks will absorb a 

 small quantity of water. The face of polished granite 

 rapidly deteriorates in severe climates, owing to the 

 freezing of the water that finds its way into the minute 

 divisions between the crystals : Cleopatra's Needle, 

 which had retained its smooth face for centuries in 

 Egypt, soon became affected after its removal to 

 London, and has to be protected by a waterproof 

 varnish, as have all the granite monuments in Canada. 



In nature also, all rocks are traversed by joints and 

 bedding planes ; these cracks are filled with water and 

 opened and extended by its conversion into ice in the 

 winter, till finally a block is wedged off and a fresh 

 surface exposed to the action. Where flagstones are 

 quarried, the workmen are in the habit of saturating the 

 surface of the rock with water before the winter sets in : 

 thus the rock is split along its bedding planes more 



