OF THE COSMOS. INTRODUCTION. 5 



mena in significant order, suggestive of their causal connec- 

 tion. The terrestrial globe was described in its form, its 

 mean density, the gradations of its temperature increasing 

 with increasing depth, its electro-magnetic currents and 

 evolution of polar light. On the reaction of the interior of 

 the planet upon its external crust depend all the phsenomena 

 of volcanic activity ; comprising those of earthquakes in 

 more or less complete circles of waves, as well as their 

 simply dynamic effects, eruptions of gas, hot springs, and 

 mud. The most powerful manifestation of internal terres- 

 trial activity is the elevation of fire-emitting mountains. We 

 have described volcanoes, both central and forming chains, 

 not only as destructive agents, but also as producing or 

 emitting various substances, and still forming under our eyes, 

 for the most part periodically, those classes of rock which 

 we term eruptive rocks ; while, in contrast with this forma- 

 tion, we have also shewn the precipitation, likewise still 

 going on, of sedimentary rocks from fluids containing their 

 minute constituent particles in solution or suspension. 

 Such a comparison of that which is still in process of elab6- 

 ration, with those strata of the crust of the globe which have 

 long since been solidified, conducts to the distinction of 

 geological epochs, and to a secure determination of the 

 successive age of formations, in which lie enveloped, in 

 successive series chronologically recognisable, the remains of 

 extinct races of plants and animals, forming the Moras and 

 Eaunas of an earlier world. The modes of formation, altera- 

 tion, and upheaving of the strata, varying at different geo- 

 logical epochs, are conditions on which all the particular 

 features of the surface of the globe depend : on them depend 

 the distribution of land and water, and the configuration and 



