INTRODUCTION. 



the presumptions of dogmatizing fancy. They 

 assure us that Encke's comet, which completes 

 its revolution in 1200 days, by reason of the 

 form and position ©f its orbit, must ever be 

 harmless to the inhabitants of the earth — as 

 harmless as Halley's comet, the great comet 

 of 1759 and 1835, with its period of 76 years ; 

 but that another comet, of shorter period, Bie- 

 la's, to wit, with its course of six years, actu- 

 ally crosses the orbit of the earth, though it 

 can only approach us nea.ly when its perihelion 

 falls at the time of our winter solstice. 



The quantity of caloric which one of the plan- 

 ets receives, and the distribution of which de- 

 termines the grand meteorological processes 

 of the atmosphere, is modified by the light- 

 evolving power of the sun — the property of its 

 surface, and the relative position of the sun 

 and the planet ; but the cyclic changes which 

 the form of the earth's orbit, and the obliquity 

 of the ecliptic, undergo, in conformity with the 

 general laws of gravitation, are so slow, and 

 confined within such narrow limits, that their 

 influence will scarcely be perceptible to such 

 instruments as we now possess for measuring 

 temperature in the course of several thousand 

 years. Cosmic causes of diminished tempera- 

 ture, of lessened fall of rain, and of epidemic 

 diseases, which were much canvassed in the 

 middle ages, and of which mention has again 

 been lately made, are consequently seen to be 

 entirely beyond the pale of actual experience. 



If I would quote other instances from phys- 

 ical astronomy, which could excite no interest 

 without a general knowledge of what has been 

 already observed, I would refer to the numer- 

 ous instances of differently coloured double 

 stars which move in ellipses round one anoth- 

 er, or rather around their common centre of 

 gravity ; to the periodical rarity of spots in the 

 sun ; to the regular appearance of innumerable 

 falling stars, which have now been the subject 

 of observation for so many years, and which 

 are in all probability planetary in their nature, 

 circulating round the sun, and crossing the 

 earth's orbit, in their course on the 12th or 

 13th of November, and also, according to later 

 observation, on the 10th or 11th of August. 



In the same way, general views of Cosmos 

 will alone enable us to perceive the connection 

 betwixt the theory of the pendulum swinging 

 in air, and the internal density — I might say, 

 the degree of congelation or solidification — of 

 our globe, a theory happily completed by the 

 acuteness of Bessel ; betwixt the production of 

 crystalline rocks in stratified streams of lava 

 upon the acclivities of still active volcanoes, 

 and the endogenous granitic, porphyritic, and 

 serpentine rocky masses, which, forced up from 

 the interior of the earth, have burst through the 

 floetz formations, and produced various effects 

 upon them— hardening or silicifying them, con- 

 verting them into dolomite, producing drusy 

 cavities, filled with crystals, &c. ; betwixt the 

 elevation of islands and conical mountains, 

 through elastic forces, and the uplifting of 

 mountain chains and entire continents — a con- 

 nection which has been acknowledged by the 

 greatest geologist of our age, Leopold von Buch, 

 and illustrated by a series of admirable observa- 

 tions. Such upheavings of granular mountain 

 masses and floetz strata, as have even lately 



been witnessed over a vast extent of the coast 

 of Chili, in connection with an earthquake, 

 show us how possible it is that the marine 

 shells, which Bonpland and I discovered on the 

 slopes of the Andes, at an elevation of 14,000 

 feet above the level of the sea, were brought 

 thither, raised from the bed of the ocean by 

 volcanic forces, not by any general flood that 

 overspread the surface as it now presents itself 

 to us. 



By Plutonism, or Vulcanism, taking either 

 word in its most general sense, and using it not 

 only with reference to the earth, but also to its 

 satellite, the moon, I mean the reaction which 

 the interior of a planet exerts upon its crust. 

 He who is unacquainted with the observations 

 that have been made on the gradual rise of tem- 

 perature, as the crust of the earth is penetrated 

 more deeply (observations which have led dis- 

 tinguished naturalists to conclude that at the 

 depth of five geographical miles below the sur- 

 face, a temperature adequate to keep granite in 

 a state of fusion prevails*'), is not prepared to 

 appreciate many recent observations on the 

 simultaneousness of the eruptions of volcanoes, 

 separated by vast extents of country, on the 

 limits of the circles within which earthquakes 

 are likely to be felt, on the permanence of the 

 temperature of hot mineral springs, and on the 

 difference of temperature of the water in Ar- 

 tesian wells of different depths. And yet this 

 knowledge of the internal temperature of the 

 earth throws a feeble light upon the primary 

 history of our planet. It proclaims the possi- 

 bility at a former epoch of the general diffusion 

 of a tropical climate over the surface of the 

 globe, as a consequence of heat inherent, arid 

 of clefts pouring forth heat, in the lately con- 

 creted and oxidated crust of the earth. It re- 

 minds us of a state of things in which the tem- 

 perature of the atmosphere may have been 

 more intimately connected with the reaction of 

 the interior upon the exterior, than with the 

 position of the axis of revolution of our planet 

 to the great central mass of our system, the 

 sun. 



Numerous productions of the tropics are now 

 dug up by eager geologists from their tombs in 

 the temperate and cooler regions of the earth : 

 coniferous vegetables, trunks of palm trees, 

 erect as when they grew, arborescent ferns, 

 goniatites, and fishes with rhomboidal pearly 

 scales, in the old coal formations(") ; skeletons 

 of colossal crocodiles, long-necked plesiosauri- 

 ans, the scales of planulites, and the stems of 

 cicadeae, in the Jura limestone ; polythalamians 

 and bryozoa in chalk, in several instances iden- 

 tical with species still existing in our seas ; 

 vast agglomerations of infusory animalcules, as 

 brought to light by Ehrenberg's all-animating 

 microscope, in beds of tripoli, semiopal and si- 

 liceous sinter (1) (Kieselguhr) ; bones of hyenas, 

 lions, and elephantine pachydermatous animals, 

 lying exposed in caverns, or covered merely 

 with a layer of sand or mud. With a compe- 

 tent knowledge of other natural phenomena, 

 these productions do not remain objects of 

 mere idle curiosity and wonder ; they become 

 more worthily the occasion of much varied and 

 interesting reflection. 



In the multiplicity of objects which I have 

 thus cursorily enumerated, the question pre- 



