PKESIIM", N'TIAL ADDRESS SliCTlON A. 33 



inch, while at 2,000 miles, or half-way to the centre, a probable 

 estimate is 8.000 tons to the square inch. It will help to gave 

 some idea of the effects of such pressures if we recollect that 

 a block of good granite, if unsupported laterally, will generally 

 begin to crush under a pressure of about 11 tons to the square 

 inch, so that a column of granite four miles high will, if devoid 

 of lateral support, begin to crumple at the base under its own 

 weight. We shall see. however, later that strength to resist 

 crushing is immensely increased if the column be given lateral 

 support. 



To complete a first rough picture of the earth's structure, 

 we must turn now to the question of the temperature of the 

 interior and of its physical state. 



As all w^ho have to do with mining are well aware, after 

 a borehole passes through the first few feet, the temperature 

 always gets higher the deeper we go. The rises observed in dif- 

 ferent boreholes and tunnels have varied considerably, but we 

 shall not be far wrong in taking a rise of about one Centrigrade 

 degree for every 100 feet, or, say. 50 degrees per mile as an 

 average. If this rate were maintained for even a few miles in 

 the crust, we see that at a depth of only 40 miles the temperature 

 would be about 2,000° C, a temperature well above the melting- 

 point of all known rocks, when these rocks are subjected to 

 ordinary pressures. 



The deduction made by most geologists and many physicists 

 till well on in the nineteenth century was that a part of the 

 interior of the earth must be liquid, and the innermost part even 

 a highly compressed gas. 



There are, however, many facts which militate very strongly 

 against the soundness of such conclusions. 



In the first place, it is, as we shall see, very doubtful whether 

 the surface gradient is maintained. Again, direct experiments on 

 substances in the laboratory show that increase of pressure raises 

 the melting-points of almost all substances, including rocks, and 

 also increases enormously their stiffness or rigidity when below 

 the melting-point. Finally, the evidence of observations on earth- 

 tides practically precludes the possibility of the existence of a 

 continuous shell of liquid or gas anywhere, and this is confirmed 

 strongly by the evidence of the propagation of earthquake waves 

 as observed by the seismometers now installed at numerous 

 places. 



On the other hand, these is strong evidence of the existence 

 of a layer beginning at some depth between 20 and 100 miles 

 beneath the surface, and probably extending considerably down- 

 wards, wdiich is relatively less rigid than the rest, and which 

 intermittently, perhaps, or like a plastic solid, adjusts itself to 

 the constantly varying burden which the surface agencies lay 

 upon it when they corrode and denude the continental areas, and 

 deposit the products along the oceanic borders. 



We shall shortly consider these points in turn. 



