84 Evolution of the Earth. 



the bowels of the earth, would at first appear to favor the 

 theory of the molten condition of the interior. The phe- 

 nomena of volcanoes and earthquakes have also been ad- 

 duced in support of this theory. At present, however, it is 

 the judgment of the best authorities in geological science 

 that the inner core of the earth, though intensely hot, is not 

 in a liquid, but in a glutinous-solid condition ; — in a pli- 

 able state, but much more dense, even, than the solid rocks 

 which we see around us. The reason for this conclusion is 

 the recognition of the effect produced by the immense pres- 

 sure exerted on the earth's interior by the superimposed ma- 

 terial. The entire weight of the earth has been estimated 

 to be about 6,000,000,000,000,000,000,000 tons — a number 

 entirely too great for the human imagination to grasp. When 

 we reflect that the atmospheric pressure, at the level of the 

 sea, amounts to fifteen pounds to each square inch of ex- 

 posed surface, we can imagine — or rather, we cannot imag- 

 ine — the immense pressure exerted on the inner core of the 

 earth by the masses of material above and around it ; suffi- 

 cient, no doubt, to compress it to a dense, viscous mass, in 

 spite of the expansive power of its extreme heat.* It is 

 only where some fissure or flaw in the superimposed strata 

 relieves it from a portion of this pressure that it liquefies 

 as it comes to the surface in volcanoes. 



Formation of Stratified Rocks. We have seen how 

 the soil is made by various processes of attrition and disin- 

 tegration from the rocks. We have now advanced so far in 

 our investigations that we can go on to study the manner 

 in which the stratified rocks have been formed. The earth's 

 floor wherever it is exposed, or as it would appear if this 

 carpet of soil and vegetation were removed, is composed of 

 a wonderful variety of formations. Think, for example, of 

 the number of kinds of stone used in building — granite, 

 marble of many varieties, limestone, sandstone, slate, many 

 others will occur to you — but these constitute only a por- 

 tion of the almost endless variety of rocks which the geol- 

 ogist has discovered and named, and all of them have been 

 produced by processes of differentiation and integration, 

 from the primitive material of the earth's crust. These 

 rocks are spread over the earth in a very irregular manner. 

 If you were to make a map of them, it would exhibit a most 



* Enormous pressure would be exerted, notwithstanding the variation in the 

 force of gravity as we approach the center of the earth. 



