4 



PROPORTION OF HABITABLE LAND TO THE 



to the animals and vegetables of tropical climates ; and hence it has 

 lieen inferred by some geologists, that the central mass of heat is grad- 

 ually refrigerating. It is, however, the crust of the globe that offers 

 proper occupation to the geologist. The greatest depth to which 

 he can extend his observations from the uppermost strata, to the 

 very lowest beds that have been raised up or laid bare by these nat- 

 ural operations which have formed mountains or valleys, is less than 

 eight miles ; a thickness which, compared with the bulk of the earth 

 itself, does not exceed that of a coat of varnish upon an artificial 

 .terrestrial globe. Were we to bear this sufficiently in mind, the 

 mighty catastrophes which have changed the surface of the globe 

 in former periods, and have left traces of their action, appalling to the 

 imagination, would cease to exceed the sober measure of belief. 



The superficies of our planet is calculated to contain about one 

 hundred and ninety millions of square miles ; but could we be rais- 

 ed to a sufficient height above the earth, so as to have its whole en- 

 lightened hemisphere for our horizon, we might perceive as it revol- 

 ved under our feet, how small a portion is fitted for the habitation 

 pf man. More than three fifths of the earth's surface are covered 

 by the ocean ; and if from the remaining part we deduct the space 

 occupied by polar ice and eternal snow, by sandy deserts, steril moun- 

 tains, marshes, rivers and lakes, the habitable portion will scarcely 

 exceed one fifth of the whole of the globe. Nor have we reason 

 to believe that at any former period the dominion of man over the 

 earth was more extensive than at present. The remaining four fifths 

 of our globe, though untenanted by mankind, are, for the most 

 part, abundantly stocked with animated beings, that exult in the 

 pleasure of existence, independent of human control, and no way 

 subservient to the necessities or caprices of man. Such is and 

 has been for several thousand years the actual condition of our plan- 

 et ; nor is the consideration foreign to our subject, for hence we may 

 feel less reluctance in admitting the prolonged ages or days of crea- 

 tion, when numerous tribes of the lower orders of aquatic animals 

 lived and flourished, and left their remains imbedded in the strata that 

 .compose the outer crust of our planet. 



The ocean has been an important agent in effecting vast changes 

 on the surface of our globe, which will be hereafter considered. The 

 average depth of the sea has been difFerendy estimated. According 

 to Laplace, this depth cannot be less than ten mil^s, to account for 

 the height of the tides by the laws of gravitation ; but it is more 

 generally admitted that the average depth does not exceed five miles. 

 jS[o admeasurement by soundings has exceeded the depth of one mile 

 and a quarter. 



The ocean has not always occupied ils present bed, for rocks al- 

 most entirely composed of the shells or remains of marine animals, 

 are found in almost every country that has yet been explored ; and 

 jihese remains occur near the summits of ihe higliest mountains, m 



