ON VOLCANIC ACTION. 4ll 
records. Volcanicity, or the influence which the 
interior of our planet exercises upon its external en- 
velope 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 toa small 
number of points ; intermittent ; simplified in its 
chemical effects; producing rocks only around small 
circular apertures, or over longitudinal cracks of 
small extent; and manifesting its power, at great 
distances, only dynamically, by shaking the crust 
of our planet in linear directions, or in spaces which 
remain the same during a great number of ages. 
Previous to the existence of the human race, the ac- 
tion of the interior of the globe upon the solid crust, 
which was increasing in volume, must have modi- 
fied the temperature of the atmosphere, and ren- 
dered 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 exterior, the relations of the 
earth to a central body, the sun, began almost ex- 
clusively 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 solidified crust of the globe, filled this 
crust with crevices, and injected it with masses and 
veins of basalt, metallic substances, and other mat- 
ters, introduced after the solidification of the planet 
had been completed. The period of the great geo- 
logical revolutions was that when the communica- 
tions between the fluid interior of the planet and its 
atmosphere were more frequent, acting upon a greater 
