20 R. D. Oldham — The Interior of the Earth. 



yet undergone these processes, but is thoroughly cooled and solid in 

 every sense of the word. To the greatest depth yet reached the 

 rocks are of this type, and it evidently continues for some distance 

 below the depths which can be reached by direct observation or by 

 immediate deduction. This outermost portion of the earth, in which 

 the physical condition is similar to that of the surface rocks, is 

 commonly known in geology as the crust, a name which originated 

 in the days when the earth was supposed to consist of a molten 

 interior and an outer solid crust, and has survived that supposition 

 for want of a better, and after all the word does not necessitate 

 a fluid interior; a loaf of bread, for instance, has a crust, though the 

 interior should be solid. At the present day the term means no 

 more than the outer layers in which composition and constitution 

 have not undergone any great change, as opposed to the more deeply 

 seated material, which differs on one or both of these characteristics. 

 The thickness of this outer crust has been estimated by various 

 methods, the increase of temperature, the pressure under which 

 certain minerals must have been formed, the strength of the crust as 

 indicated by the anomalies of gravity, and some others, all of which 

 agree in putting the lower limit at about 50 km., or some 30 miles, 

 well under one-hundredth' of the radius of the earth. 



In the outer regions of the crust geological investigation has 

 shown that movements of displacement have taken place on a large 

 scale. Over widths of some hundreds of kilometres rocks have been 

 compressed to the extent of one-third to one-half or less of their 

 original extension, as is shown by the folds into which originally 

 horizontal strata have been thrown. In other cases clean-cut, 

 gently sloping, or horizontal fractures have been recognized, and, 

 along these fractures, displacements have taken place to the extent, 

 well established, of over 60 km. and in some cases possibly of over 

 double this, or from ^° to 1° of the circumference of the earth. 

 In other places there is evidence of extension, much less capable of 

 measurement than, the compression, which, though probably smaller 

 on the whole than the compression, may be comparable in amount. 

 Displacements in a vertical direction have also been well determined, 

 along defined surfaces of fracture the displacement of opposite sides 

 of faults has been established up to about 5 km., and possibly to half 

 as much again, and, as regards places less than a couple of degrees 

 apart, such as the crests of the Himalayas or Andes compared with 

 the plains at their foot, or the mountains of Japan with the bottom 

 of tbe Tuscarora deep, the vertical displacements may be as much 

 as 15 km. 



The cause of these great earth movements is still an unsolved 

 problem of geophysics. At one time they were generally attributed 

 to compression of the earth's crust through contraction due to gradual 

 cooling, and the notion is by no means extinct, but the curiously 

 local distribution of tbe compression is against this interpretation, no 

 less than the fact that the amount is largely in excess of any con- 

 traction permissible on this hypothesis, and besides we have the 

 equally well-established facts that regions of very considerable 

 extent show signs of tension and expansion of their dimensions. 



