26 



THE EARTH FORMED. 



greater at greater depths ; so that the entire mass of a 

 cool globe should be of a gravity infinitely exceeding 

 four and a half times the weight of water. The only al- 

 ternative supposition is, that the central materials are 

 greatly expanded or diffused by some means; and by 

 what means could they be so expanded but by heat ! In- 

 deed, the existence of this central heat, a residuum of 

 that which kept all matter in a vaporiform chaos at first, 

 is amongst the most solid discoveries of modern science,* 

 and the support which it gives to Herschel's explanation 

 of the formation of worlds is most important. We shall 

 hereafter see what appear to be traces of an operation of 

 this heat upon the surface of the earth in very remote 

 times ; an effect, however, which has long passed entirely 

 away. The central heat has, for ages, reached a fixed 

 point, at which it will probably remain for ever, as the 

 non-conducting quality of the cool crust absolutely pre- 

 vents it from suffering any diminution. 



THE EARTH FORMED— ERA OF THE PRIMARY 

 ROCKS. 



Although the earth has not been actually penetrated 

 to a greater depth than three thousand feet, the nature 

 of its substance can, in many instances, be inferred for 

 the depth of many miles by other means of observation. 

 We see a mountain composed of a particular substance, 

 with strata, or beds of other rock, lying against its sloped 

 sides : we, of course, infer that the substance of the 

 mountain dips away under the strata which we see lying 

 against it. Suppose that we walk away from the moun- 

 tain across the turned up edges of the stratified rocks, and 

 that for many miles we continue to pass over other stra- 

 tified rocks, all disposed in the same way, till by and bye 

 we come to a place where we begin to cross the opposite 

 edges of the same beds ; after which we pass over these 

 rocks all in reverse order, till we come to another exten- 

 di sive mountain composed of similar material to the first, 

 and shelving away under the strata in the same way. We 

 should then infer, that the stratified rocks occupied a ba- 

 * The researches on this subject were conducted chiefly by the 

 late Baron Fourier, perpetual secretary to the Academy of Sciences 

 of Paris See his Theorie Jlnalytique de la Chaleur, 18-22. 



