Volcanos. 121 



of lava torn from the surface of that which has risen within the 

 vent, by the explosive escape of the ascending volumes of 

 steam. The distinctions of shape, structure and size, are ac- 

 counted for, particularly the difference of the scoriae of feld- 

 spathose lavas (pumice,) and those of basaltic composition. 2. 

 Fragments of other rocks broken from the sides of the fissure 

 by the force of the ascending fluids. These may consist of 

 primitive, secondary, or any class of rocks. The remarka- 

 ble fragments found in the conglomerates of Somma, and Eif- 

 fel, 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 re- 

 peated projections. The electric phenomena, developed du- 

 ring 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 projec- 

 ted a mass of rock of 1000 cubic feet to a distance of three 

 leagues. This explosive force proves the vapor 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 struc- 

 ture, the character of such a hill may be recognized even from 

 the smallest remaining 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 disposi- 

 tion of lavas, when expelled, en masse, in a more or less fluid 

 state from the volcano ; beginning with a notice on the mineral na- 

 ture 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 min- 

 eral, 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, al- 

 most the sole ingredient. The fluidity of a lay,a, or the facility 

 with which it moves in 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 tempe- 

 rature. Hence lavas, of the same mineral quality, and there- 

 fore of equal specific gravity, when produced under similar cir- 

 cumstances of temperature and pressure, will possess a degree 

 Vol. XIII.— No. 1. 16 



