(41 ) 



THE EARTH FORMED. 



EEA OF THE PRIMArtY ROCKS. 



ALTnouc4ii 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 

 mountain across the turned up edges of the stratified 

 rocks, and that for many miles we continue to pass over 

 other stratified rocks, all disposed in the same way, till 

 by-and-by 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 extensive 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 basin formed by the rocks of these 

 two mountains, and by calculating the thickness right 

 through these strata, could say to what depth the rock 

 of the mountain extended below. By such means, the 

 kind of rock existing many miles below the surface can 

 often be inferred with considerable confidence. 



