396 Scientific Intelligence. 



sive eruptions may lose its head ; but if so, it is by swallowing 

 it, or simply by a collapse. The same is the process in quiet 

 Kilauea : the solid lavas of the borders of the fiery region sink 

 because the discharge of the liquid rock makes a void beneath 

 them. 



Vesuvius, through the viscidity of its lavas and its sources of 

 vapors, has at times vast projectile force, and much is often attri- 

 buted to its blowing powers ; as in such sentences as this: "broke 

 up and blew into the air the whole upper part of the cone."* But 

 its processes are in the main, as in Kilauea, (1) filling, (2) dis- 

 charging, (3) collapsing. It alternates in its conditions between 

 a volcano with a profound crater and one with a top plain — the 

 " altopiano" of Italian writers. The crater pit (sometimes 1000 

 feet deep) slowly fills by small ejections within it of lava and 

 cinders.f Outflows of lavas from fissures in the mountain's side 

 may occur at different stages in the filling ; but usually the work 

 goes on, with no great outbreak, until the cavity is full and a 

 top-plain of solidified lavas, a mile or more in circuit, takes its 

 place, with nothing left of the crater except the vent of a cinder- 

 cone. If the fires now increase in activity, the plain may mostly 

 disappear by the formation over it of a large cone (or two or 

 more of them) about an area (or areas) of boiling lava throwing 

 up lapilli, lava-masses, and bombs from the vent. Then, if an 

 outbreak of the grander kind takes place, discharge upward of 

 lava-fragments to great heights accompanies discharge outward- 

 and-downward of lavas through opened fissures ; and a collapse 

 results owing to the loss by the upthrow and outflow ejections ; 

 consequently, what is left of the top-plain and its cones, with 

 sometimes an upper portion of the great cone disappears by sub- 

 sidence. Thus it happened at the eruption of May 31st, 1806 ; 

 for Signor G. Zorda, speaking of it, says: "a considerable part of 

 the summit fell into the volcanic abyss ;"| and facts enough are 

 reported to show that the same took place at the grander eruption 

 of 1822. In the smaller eruptions of Vesuvus the "altopiano" 

 often undergoes little change, because the undermining is not 

 sufficient for more ; and after a while cinder-eruptions may be 

 resumed ; consequently after such eruptions, as Scacchi observes, 

 the height of Vesuvius may become increased. 



The writer, at a visit to Vesuvius in July, 1834, found the 

 summit a plain — the altopiano — with a small spurting cinder-cone 

 near its center. A red heat existed 10 to 20 inches down in 

 many fissures over the plain, and at one place a stream of lava, 

 4 to 5 feet wide, emerged and flowed off down the mountain. § 

 A month later, in August, a great eruption occurred, the lava 



* Scrope on Volcanoes, p. 17. 



f Well described by Scacchi for the interval 1840 to 1855, in his Eruzione 

 Vesuviane del 1850 e 1855, Napoli, 1855. 



% Relazione, etc., Napoli, L806, in Scacchi's Memoir on Vesuvius, 1855, p. 15. 



§ On the condition of Vesuvius in July, 1834, this Journal, xxvii, 281, 1835. 

 For a sketch of the cone as it then appeared, see the writer's Text-book of Ge- 

 ology, p. 130. 



