116 R. M. Deeley — Mountain Building. 



cube would dSform in some such way as this, and the deformation 

 would never become appreciable. 



However, if a soft solid have too great a weight placed upon it, it 

 will yield continuously much as will a liquid. In extruding lead 

 through a hole in a cylinder no movement takes place until a certain 

 pressure is reached; the lead then flows out through the hole and 

 flows faster as the pressure is increased. 



Now the upper portion of the earth's surface is composed of rocks 

 varying in hardness. Buildings may be erected upon it which will 

 be permanently stable, provided the load on their foundations is not 

 too great. The load per square foot that may with safety be placed 

 upon each kind of rock varies considerably, and it is the engineer's 

 endeavour to find out what is the smallest area and depth of founda- 

 tion that will ensure stability in each case, so as to keep the cost of 

 the building as low as possible. 



The load-carrying capacity of any kind of rock varies much 

 according to surrounding conditions. Dry clay will carry more than 

 wet clay. On this account the water escaping from a burst water- 

 pipe may so soften the clay or marl upon which a building rests 

 that the foundations give way. Vibrations caused by heavy vehicles 

 have the same effect. Railway bridges and retaining walls have 

 to be made much stronger, for the same loads, than have road bridges 

 and walls. No doubt the foundations of St. Paul's Cathedral were, 

 in most instances, amply sufficient under the conditions existing 

 when Wren built it, but the vibrations resulting from the heavy 

 road and rail traffic of our times, and the effects upon the local 

 drainage produced by sewers, etc., have much interfered with their 

 stability. 



The nature of the resistance to stress offered by the materials 

 forming the earth's crust is in some cases of a liquid and in others of 

 a plastic description. The liquids, whether water or molten rock,, 

 settle down with their npper surface layers practically horizontal ; 

 but owing to their varying density and the plastic resistance they 

 offer to flow, the solid rocks stand at various levels. We thus have 

 large raised areas floating upon "roots" of light material and 

 depressed areas over roots of heavy material, the whole floating upon 

 a plastic or liquid substratum. It has been suggested that this 

 substratum rests upon harder material, and it has been called the 

 asthenosphere. But the raised areas are not necessarily quite stable. 

 The materials of which they are built up are being denuded and 

 carried to lower levels, with the result that the area rises to restore 

 the balance, and the areas over which deposition takes place sink for 

 the same reason. 



The flexing of the earth's crust by the moon results in "earth 

 tides ", and these, together with earthquake shocks, have the same 

 effect upon the stability of mountain masses as have the vibrations 

 produced by heavy traffic upon the foundations of St. Paul's 

 Cathedral. Elevated areas consequently tend to flow and spread 

 outwards over the surrounding lower lands, or to slide under the 

 action of gravity bodily into depressions. Such sheets of rock are not 

 pushed along by pressure applied at one end ; they slide bodily down 



