342 Analysis of' Scientific Books and Memoirs, 



the ascending fluids. These may consist of primitive, secondary, or any 

 class of rocks. The remarkable fragments found in the conglomerates of 

 Somma, and the Eiffel, are attributed to the alteration of calcareous and 

 granite fragments by the volcanic heat, new minerals being produced from 

 the decomposition of these, and the reaggregation of their elements in 

 other forms. Fragments of either kind are, if the eruption continues 

 long enough, completely pulverized by repeated projections. The electri- 

 cal phenomena, developed during eruptions, are supposed by the author to 

 be owing to this immense friction. The height to which fragments of a 

 large size are carried, exhibits the prodigious escaping force of the steam- 

 bubbles. Vesuvius has been seen to launch scoriae 4000 feet above its 

 apex, Catopaxi 6000. The latter projected a mass of rock of 1000 cu- 

 bic feet to a distance of three leagues. This explosive force proves the 

 vapour to be propelled from a great depth, and at an intense heat. A 

 volcanic cone is shown to be stratified in planes parallel both to the inner 

 and outer slopes of the hill ; and, owing to this peculiarity of structure, 

 the character of such a hill may be recognized even from the smallest re- 

 maining fragment. A plate gives a view of the Capo de Miseno, which 

 offers a natural section of such a cone. The author next discusses the 

 laws of the protrusion and disposition of lavas, when expelled, en masse, 

 in a more or less fluid state from the volcano; beginning with a notice on 

 the mineral nature of lavas, and their differences of specific gravity and 

 texture, by which their fluidity is invariably determined. He classes 

 them into the heavier lavas (basalt,) in which the ferruginous minerals, 

 augite, hornblende, mica, or titaniferous iron, are abundant; and the 

 light lavas (or trachytes,) in which the minerals are rare, and felspar, or 

 some equivalent of low specific gravity, almost the sole ingredient. The 

 fluidity of a lava, or the facility with which it movesjn obedience to its own 

 gravitating force, is compounded of its liquidity, of the mobility of its parts, 

 and of its specific gravity. But the liquidity of lavas, it has been seen before, 

 varies with the average comminution of their crystalline particles, under 

 the same circumstance of pressure and temperature. Hence lavas, of the 

 same mineral quality, and therefore of equal specific gravity, when pro- 

 duced under similar circumstances of temperature and pressure, will possess 

 a degree of fluidity inversely proportioned to the average size of their crys- 

 talline particles, or grain. And, vice versa, when their grain is of the same 

 degree of fineness, their fluidity will be proportioned to their specific gra- 

 vity, i. e. to the proportion of ferruginous minerals in their composition. 



The author illustrates these propositions, by showing, from observation, 

 that the basaltic lavas have generally flowed farther, and spread over a 

 larger surface than the trachytic ; and also, that the lavas of either class 

 have spread in a horizontal direction, more or less, in proportion to the 

 abundance of the heavier minerals in their composition, and the fineness 

 of their grain. The fact has been long ago remarked, but the explanation 

 of it is presumed to be novel. The disposition of a body of lava, emitted 

 from an orifice in the surface of the earth, is, in strictness, determined by, 

 1. The force of expulsion— 2. Its fluidity— <3. The external circumstances 



