18 THE CRUST WE DWELL UPON. 



basins of the continents and islands. To this rocky ex- 

 terior geologists apply the term " crust," much in the same 

 way as the housewife speaks of the crust of her loaf, or the 

 schoolboy of the crust of ice that forms on the stagnant 

 pool during the frosts of winter. The crust is something 

 hard and consistent, and may differ both in nature and 

 consistency from the interior on which it rests, or over 

 which it may be formed; and this is precisely the idea 

 entertained by geologists when they speak of the outer 

 shell or " crust of the globe." 



The rocky exterior over which we travel, and into which 

 we dig and mine and tunnel, is a thing we can see and in- 

 vestigate to a limited depth ; but the interior, sinking away 

 four thousand miles to the centre, is placed altogether 

 beyond our reach and observation. It may consist of rocky 

 substances, but if so, they must be in a condition as to 

 density altogether different from those we find at the sur- 

 face ; for as a planet the earth has a certain astronomically- 

 ascertained weight, and were the force of gravitation to 

 exert itself to the centre on such rocks as we know, their 

 compression would give to the earth's mass a weight far 

 exceeding that which its astronomical relations will allow.* 

 Again, as we descend into the earth by mines, shafts, and 

 Artesian wells, the temperature seems to increase at a given 

 ratio (about one degree Fahr. for every 60 feet of descent); 

 and at this rate a depth would soon be reached at which 

 every known substance would be held in a state of incan- 

 descent fusion, or even vaporiform dispersion. It is conve- 

 nient, therefore, to draw a distinction between the " crust" 

 we can examine, and the " interior," respecting which we 



* The reader must guard against the idea that at extreme depths 

 all substances suffer alike from mutual mechanical pressure. Their 

 different compositions forbid this supposition ; and their densities must 

 continue to depend (no matter what the depth) more on their chemical 

 nature than on the amount of compression to which they are subjected. 



