Mr Poulett Scrope's Considerations on Volcanos. 341 



activity in tempestuous weather, and, in general, to be most violent in the 

 stormy season of the year. Earthquakes also have been often observed to 

 coincide in time with hurricanes or violent storms ; and the author notices 

 the probability that a diminution of the pressure of the atmosphere, 

 taking place simultaneously on a large extent of the earth's surface, be- 

 neath which the ever-active force of expansion is continually pressing up- 

 wards, and often restrained by only the slightest degree cf superiority in 

 the combined forces of repression, may occasionally give the predominance 

 to the former force, and determine one of those partial elevations of the 

 crust of the globe to which he attributes the phenomena of earthquakes. 



It is remarked, that an individual volcano may occasionally pass from 

 one of the phases, distinguished above, into another ; or may even exist in 

 two phases at once, having a double system of operations, corresponding 

 to two different foci, seated one below the other ; the latter perhaps at a 

 considerable elevation in the chimney or main vent of the volcano, and 

 giving rise to minor and frequent eruptions ; the former at a much great- 

 er depth, and productive of rare and violent paroxysmal eruptions. It is 

 obvious that the last system must be in activity wherever the supply of 

 caloric is in a faster ratio than its drain through the activity of the upper 

 focus. The paroxysmal eruptions leave usually a prodigiously wide and 

 deep erater, which is subsequently filled up by degrees by the erup- 

 tions of the minor and upper focus. Such alternations of minor and pa- 

 roxysmal eruptions appear to have produced the cokssal cratei-al cavities 

 of volcanic countries, most of which have one or more recent cones rising 

 from within their circuit, such as Vesuvius within the crater of Somma; 

 the Peak of Teneriffe, and the cone of Chahorra, from the circus describ- 

 ed by Von Buch ; that of Bourbon from the successive circuses described 

 by St Vincent; those of Volcano, Astroni, the lake of Roneiglione, &c. &c. 

 The author now proceeds to examine the laws which determine the dis- 

 position of voleanic products on the surface of the globe. The simple 

 cone is first considered, resulting from the accumulation of fragments pro- 

 jected by a series of explosions from a single aperture. Its figure is a 

 truncated cone, containing a funnel-shaped cavity called the crater. The 

 line in which the inner and outward slopes meet is the ridge. Its regula- 

 rity is liable to disturbance from many causes, such as the fissure-like 

 form of the vent, which usually gives an oblong figure to the cone; the 

 vicinity of other vents; violent prevailing winds; and, above all, the sub- 

 sequent emission of a current of lava from the same orifice by which one 

 side of the crater is broken down. Examples of these, and other varieties 

 of figure, are given from Auvergne, Italy, &c. 



The composition of the cone is next dwelt on, and the nature of the 

 fragments, which are either, I. Scoria, or portions of lava torn from the 

 surface of that which has risen within the vent, by the explosive escape 

 of the ascending volumes of steam. Their distinctions of shape, struc- 

 ture, and size, are accounted for, particularly the difference of the scoriie 

 of feldspathose lavas (pumice,) and those of basaltic composition. <2. Frag- 

 ments of other rocks broken from the sides of the fissure by the force of 



