EARTH'S SURFACE METEORIC ABRASION. 83 



while the wind clears out and carries away the 

 particles as fast as they are formed; thereby 

 causing what was once solid cliffs of rock to have a 

 built appearance. That heat and wind unaided can. 

 thus work is proved by weathered rocks occurring 

 in rainless regions. In many places in our own 

 climate, but more especially those exposed to sea or 

 mountain blasts, work somewhat similar, except that 

 it is on a much smaller scale, can be studied. 

 Wind work evidently has a great deal to do with the 

 formation of those peculiar rock masses that have 

 been called by such names as " mushroom "- and 

 " table-rocks ; " as it drives the rain under them, also 

 into the crack and joints, to be there contracted and 

 expanded by heat and cold, which disintegrates the 

 moistened portions, while subsequently the wind 

 again acts and carries away the particles. It may be 

 asked, Why do not these agents act on the whole 

 surface of the rocks ? So they do, but necessarily the 

 maximum work must be in the immediate vicinity of 

 joints and other similar lines, opening them and 

 weathering away the adjoining parts of the rock. 

 " Mushroom "-rocks are also formed by wind in con- 

 junction with the waters of a lake, when the joints or 

 other lines have very little effect, as the rocks must be 

 worn away at heights corresponding with the ordinary 

 summer and winter levels of the waters. Chemical 

 action must here assist considerably, especially if the 



