418 COSMOS. 



very complicated variations, in the most distant quarters of 

 the globe than in the previous century, when the entire mor- 

 phology of volcanoes was limited to conical and bell-shaped 

 mountains. There are many volcanoes whose configuration, 

 altitude, and range (what the talented Carl Friedrich Nau- 

 mann calls the geotectonics)* we now know in the most sat- 

 isfactory manner, while we continue in the greatest ignorance 

 regarding the composition of their different rocks and the 

 association of the mineral species which characterize their 

 trachytes, and which are recognizable apart from the princi- 

 pal mass. Both kinds of knowledge, however — the morphol- 

 ogy of the rocky piles and the oryctognosy of their compo- 

 sition — are equally necessary to the perfect understanding of 

 volcanic action ; nay, the latter, founded on crystallization 

 and chemical analysis, on account of the connection with 

 Plutonic rocks (porphyritic quartz, green-stone, and serpent- 

 ine) is of even greater geognostic importance. The little we 

 believe we know of what is called the volcanicity of the Moon 

 depends too, from the very nature of the knowledge, on con- 

 figuration alone.f 



* The fullest information we possess on any volcanic district, found- 

 ed on actual measurements of altitudes, angles of inclination, and 

 profile views, is contained in the beautiful work of the astronomer of 

 Olmutz, Julius Schmidt, on Vesuvius, the solfatara, Monte Nuovo, the 

 Astroni, Rocca Monfina, and the old volcanoes of the Papal territory 

 (in the Albanian Mountains, Lago Bracciano, and Lago di Bolsena). 

 See his hypsometrical work, Die Eruption des Vesuvs im Mai, 1855, 

 with Atlas, plates iii., iv., ix. 



f The progressive perfection of our acquaintance with the formation 

 of the surface of the Moon as derived from numerous observers, from 

 Tobias Mayer down to Lohrmann, Madler, and Julius Schmidt, has 

 tended, on the whole, rather to diminish than to strengthen our belief in 

 great analogies between the volcanic structures of the earth and those 

 of the moon ; not so much on account of the conditions of dimension and 

 the early recognized ranging of so many ring-shaped mountains, as on 

 account of the nature of the rills and of the system of rays which cast 

 no shadows (radiations of light) of more than 400 miles in length and 

 from 2 to 16 miles in breadth, as in Tycho, Copernicus, Kepler, and 

 Aristarchus. It is remarkable, however, that Galileo, in his letter to 

 Father Christoph Grienberger, Sidle montuosita delta Lima, should have 

 thought of comparing annular mountains, whose diameters he consid- 

 ered greater than they actually are, to the circumvallated district of 

 Bohemia, and that the ingenious Robert Hooke, in his "Micography," 

 attributes the type of circular formation almost universally prevalent 

 on the moon to the action of^the interior of its body on the exterior 

 (vol. ii., p. 701, and vol. iv., p. 496). With respect to the annular 

 mountain ranges of the moon, I have been of late much interested 

 with the relation between the height of the centi'al mountain and that 

 of the circumvallation or margins of the crater, as well as by the exist- 



