OF SURFACE. 179 



The external crust, or covering of the earth, is composed of a 

 vast amount of substances which we shall more fully describe 

 hereafter, but which, under the indefinite but convenient terms 

 of rocks and earth, embracing every variety of element, and 

 combination, are familiar to every one.- Although of such mi- 

 croscopic value as regards the dimensions of the globe itself, yet 

 the crust upon which we are located, is of infinite importance to 

 man. With its alternations of land and water,- of valleys and 

 mountains, it is the seat of vast empires, and the storehouse of 

 the wealth of nations. The surface of the earth has been com- 

 puted to contain one hundred and fifty millions of square miles, 

 about three-fourths of which are covered by seas, and another 

 large proportion by bodies of fresh water, by polar ice, and eter- 

 nal snows ; so that, taking into the estimate the sterile tracts, the 

 forests, the barren mountains, the bogs, morasses, &c<, scarcely 

 more than one-fifth of the globe is fit for the habitation of man. 

 The area of the Pacific ocean alone, is estimated as equal to the 

 whole surface of the dry land, hence, if the waters of the globe 

 were uniformly distributed over its surface, the inequalities being 

 leveled, the whole earth would be covered with water to a depth 

 of about three feet. The presont arrangement of continents and 

 islands cannot therefore be supposed to have always existed, in- 

 deed, there is abundant evidence to show that all those parts, 

 which we call dry land, have at some very remote period been 

 underwater, and that the soil upon which we now tread, is com- 

 posed of regular strata, deposited by water. It is but a short period 

 since the utmost ignorance prevailed as to the structure of the 

 planet which we inhabit. It was accustomed to be looked upon 

 as a mass of confusion, the chaos of old, where, in incongruous 

 masses, were heaped the various substances of which it was 

 composed, and where antagonistic forces were striving confusedly 

 together. 



It was true that rocks were found at some places upon the sur- 

 face, and not at others, but this was regarded as mere matter of 

 chance, no one supposed any order, or any definite arrangement. 

 It was reserved for modern science to show that the crust of the 

 earth from its surface downwards, is composed of regular Btra- 



