144 DISINTEGRATING ACTION OF THE ATMOSPHERE. 



the wind. One of the most interesting examples of wind action is 

 recorded from the harbour of Wellington, on Cook's Strait, in New 

 Zealand, where, in a line of low sand hills, are sand-worn stones in 

 every stage of rounding. The prevailing winds drive a cloud of sili- 

 cious particles from one set of dunes to the other until their angles 

 are entirely removed, and they become rounded like the grains of sand 

 in deserts. In this country, ^Eolian action is admirably seen in the 

 pinnacles and crags on the top of Kinder Scout, a tableland of lower 

 carboniferous rocks, on which pillars of sandstone are left, which often 

 stand up in the shape of gigantic clubs or mushrooms. 



Waste of Felspathic Rocks. The exterior of most uncrystalline 

 rocks and buildings is slowly eaten away by the moisture and carbonic 

 acid of the air ; but the influence of this destructive agent is most 

 remarkable among the felspathic rocks, whether they were origi- 

 nally crystalline, like granite, or compact, like basalt. The felspathic 



Pig. 46. Millstone Grit, Yorkshire. 



portion of the hypersthene rocks of Carrock Fell is so wasted that the 

 crystals of hypersthene and magnetic iron project from the surface 

 considerably. Some greenstone dykes are thus entirely decomposed 

 to great depths from the surface, and whole masses of rotten granite 

 wait only for an earthquake or aqueous action to be entirely reduced 

 to fragments. Those who have seen the crumbled granite of Mun- 

 caster Fell in Cumberland, or Castle Abhol in Arran, surrounded by 

 heaps of its disintegrated ingredients, must have been struck by the 

 importance of this phenomenon in reasonings concerning the origin of 

 many stratified rocks. 



Both carbonic acid and oxygen act very decidedly upon the metaJ - 

 lie, and particularly the ferruginous ingredients of rocks, and thus 

 swell and burst them to pieces. Sometimes, however, this very cause 

 seems to harden and bind the rock together, and to render it more 

 durable. In general there is no certain test of the durability ol 



