4 THE STRUCTURE OF THE EARTH. 



others; and although strongly supported by Thomson for some fourteen 

 years it was abandoned by him in 1876,* and is now generally given 

 up. 



The view that the phenomena of the tides prove the earth to be solid 

 is still sustained by Thomson and Darwin, but their conclusions only apply 

 to the assumed globes and not to the earth itself Their conclusions are also 

 opposed by Hennessy, Fisher, Airy, and many others. 



The difficulty seems to be that it is beyond the power of any known 

 transcendental mathematics to grasp the problem of the earth's structure, if 

 its most probable condition be assumed. This condition may be described 

 as being that of a globe having a density gradually increasing from the 

 exterior inwards towards the centre, but with its materials heterogeneously 

 arranged, and with the lighter crust gradually and irregularly passing into 

 the heavier liquid beneath. 



If our attention be now turned to a consideration of the evidence derived 

 from the behavior of matter under the combined action of heat and pressure, 

 which behavior is said to prove that the interior of the earth is solid, the 

 important questions are : 1°. What are the materials forming the earth's 

 mass? 2°. Do these expand or contract on passing from the liquid to the 

 solid state ? 



In answer to the first question, it may be said that the results of petro- 

 graphical study render it probable that the portion of the interior mass 

 lying nearest the centre, and concerning which we have any data, is com- 

 posed of iron,t either with or without nickel. As Ave recede from this portion 

 we find pyrrhotite united with the nickel and iron. Then these minerals 

 are further joined with olivine, or olivine and enstatite, in varying propor- 

 tions, until a region is reached composed almost entirely of one or both of 

 these silicates with or without diallage. From this we pass into the common 

 basaltic rocks, then into the andesites, and so on outward into the trachytic, 

 rhyolitic, and jaspilite forms. However true this order may have been for 

 the liquid earth, it is certain that in the solid portions of the crust these 

 materials are interlaced now with each other in every conceivable way, and 

 that in the chemical and sedimentary deposits they have been intimatelj^ 

 mingled. As to what may be the composition of the earth's mass nearer 

 the centre, if there be anything there besides the iron and nickel, we have 



* Report Brit. Assoc. 1870, xlvi. (sect.) 1-12. 



t Whitney's Metallic Wealth of the United States, 1854, p. 43i. Jiidd's Volcanoes, 1881, pp. 307-324. 



