130 LITHOLOGICAL CHARACTERS 



usually blue, owing to a small quantity of iron in the 

 state of ferrous compounds. The reddish-brown layer 

 so often seen. on the broken surface of a large block of 

 sandstone is produced by the oxidation of the ferrous 

 compounds and the formation of a brown hydrated 

 sesquioxide. This is slowly removed from the outer 

 surface, so that a narrow band of light grey or white 

 rock lies between the brown band and the exterior. 

 The red stains so often seen on the sandstones are de- 

 posits of this red oxide of iron. 



The sandstone has generally a very rough surface, 

 frequently hollowed out so that it is covered with small 

 and large projections, between which are shallow de- 

 pressions that hold water for some time after rain. 

 Particles of sand collect in these and give the depression 

 a smoother surface than it otherwise would have had, 

 by being moved about in.it by strong winds. The 

 gradual lateral growth of the hollows on steeply in- 

 clined surfaces of sandstone may eventually give rise 

 to a perforation, or small arch, by meeting a joint plane 

 or a second depression formed on another surface of 

 the rock. 



The sandstone is very much jointed ; and as the pro- 

 cesses of weathering naturally go on more easily along 

 joint planes than elsewhere, for the loosened grains are 

 soon removed by the rain or wind, the large exposed 

 surfaces of sandstone are usually divided up by two or 

 more sets of deep cracks, to which another group is 

 added if the beds are so steeply inclined that the bedding 

 planes make a high angle with the ground. Where 

 these cracks become deeply eroded and are set at close 



