356 GENERAL OBSERVATIONS 



many problems in geology and physics which had 

 previously been considered inexplicable. The analo- 

 gies of observed facts, and the strict investigation 

 of phenomena of recent occurrence, gradually lead 

 us to more probable conjectures as to the events of 

 those remote periods which preceded historical 

 records. Volcanicity, or the influence which the inte- 

 rior of our planet exercises upon its external envelope 

 in the various stages of its refrigeration, on account 

 of the unequal aggregation in which its component 

 substances occur, is, at the present day, in a very 

 diminished condition ; restricted to a small number 

 of points ; intermittent ; simphfied in its chymical 

 eflfects ; producing rocks only around small circular 

 apertures, or over longitudinal cracks of small ex- 

 tent ; and manifesting its power, at great distances, 

 only dynamically, by shaking the crust of our planet 

 in linear directions, or in spaces which remam the 

 same during a great number of ages. Previous to 

 the existence of the human race, the action of the 

 interior of the globe upon the solid crust, which was 

 increasing in volume, must have modified the tem- 

 perature of the atmosphere, and rendered the whole 

 surface capable of giving birth to those productions 

 which ought to be considered as tropical, since, by 

 the effect of the radiation and refrigeration of the ex- 

 terior the relations of the earth to a central body, 

 the sun, began almost exclusively to determine the 

 diversity of geographical latitudes. 



In those primeval times, also, the elastic fluids, 

 the volcanic powers of the interior, more energetic 

 perhaps, and with more facility traversing the oxi- 

 dated and sohdified crust of the globe, filled this crust 

 with crevices, and injected it with masses and veins 

 of basalt, metallic substances, and other matters, 

 introduced after the solidification of the planet had 

 been completed. The period of the great geologi- 

 cal revolutions was that when the communications 

 between the fluid interior of the planet and its atmo- 



