INTRODUCTION. 23 



cepting the not unfrequent occurrence of long ranges 

 of cliffs^ pebbly beaches, and accumulations of sand. 

 When the coast is low and flat, the depth of the sea 

 in its vicinity is usually small ; whilst along a rocky 

 and abrupt shore it generally presents a depression 

 in some measure corresponding to the height of the 

 land. The existence of submarine chains of moun- 

 tains is established by the numerous shoals and 

 rocks which are to be considered as their summits. 

 On these, coral reefs and islands have been gra- 

 dually raised by myriads of zoophytes. 



The mighty mass of waters, which is collectively 

 termed the sea, occupies, as has been already men- 

 tioned, more than two-thirds of the surface of the 

 globe. Its chemical composition, its tides, its cur- 

 rents, and all the varied phenomena which it pre- 

 sents, afford subjects of highly-interesting research. 



Tlie atmosphere, in like manner, which envelopes 

 the earth, supplies, in its ever-varying aspects, its 

 motions, its electrical phenomena, and the influence 

 which it exercises on animal and vegetable life, an 

 object of investigation pregnant with curious and 

 useful knowledge. 



The mysterious agency of subterranean fire has 

 elevated great masses of rocky matter in various 

 parts of the globe. Earthquakes have effected ex- 

 tensive and remarkable changes upon its surface ; 

 the waters of the ocean have alternately worn away 

 the shores and eked them out by depositions of 

 sand and mud ; the rivers have furrowed the land, 

 and carried the debris of the higher regions to the 

 valleys and plains ; while air and moisture have exer- 

 cised their decomposing influence upon the hardest 

 substances. By the action of these powers the earth 



