24 INTRODUCTION. 



has become a fit receptacle for the varied forms of 

 animal and vegetable existence with which we see 

 it so profusely stored. 



The variable distribution of heat has produced 

 a striking effect in modifying the earth's surface. 

 The cold of the polar regions covers them at all sea- 

 sons with an extensive deposite of snow and ice, 

 the margins of which are periodically dissolved by 

 the increasing warmth of summer, to be repaired 

 during the succeeding winter. The numberless 

 icebergs, originally formed on the land or in its vi- 

 cinity, floating on the ocean, and drifted by winds 

 and currents, often pass into more genial regions, 

 producing occasional variations of temperature. The 

 elevated ridges of mountains experience a similar 

 degree of cold, and in all climates, even in the tor- 

 rid zone, are covered towards their summits with 

 perennial snow. 



Limited as are our powers of examining the inte- 

 rior of the globe, we yet find in its crust indications 

 of a power which, by operating so as to produce 

 apparent confusion, has effected results highly be- 

 neficial to the beings by whom the earth has been 

 peopled. The strata, at first regularly superimposed 

 upon each other, and consisting of those diversified 

 materials which are supplied by the disintegration 

 of pre-existing rocks, have been broken up, and in- 

 clined in every possible degree, so as to form those 

 depressions and elevations which we every where 

 observe on the surface. These inequalities have 

 been increased by the protrusion of masses from the 

 more central regions, and the whole has been sub- 

 jected to the agency of powerful currents of water, 

 by means of which the angular cavities and projee- 



