September 20, 1906] 



NA TURE 



527 



JHE PHYSICAL GEOGRAPHY 

 VOLCANOES.' 



OF 



T \ my discourse this evening I shall confine myself lo 



that branch of vulcanology to which I have paid 



particular attention, viz. the naked-eye study of volcanoes 



portions Atrio del Cavallo and V'alle d'Inferno, while 

 Vesuvius, the present active cone, occupies the southern 

 part of the great crater which was formed by the destruction 

 of that side of the Somma crater ring, probably in 

 the Plinian eruption which destroyed Pompeii and Hercu- 

 laneum in a.d. 79, or possibly in some earlier unrecorded 

 eruption. The observatory is situated 

 on a projecting spur forming part of 

 the old .Somma ring, but separated 

 from that mountain by a deep valley, 

 the Fossa Vetrana, at the upper part 

 of which took place the prolonged 

 eruption which lasted from 1895 to 

 i8g8, and which built up a consider- 

 able hill, the Colle Umberto 1°. 

 Photographs of this place, taken in 

 1888, showed the scoriaceous or 

 cindery lava of 1872, couMes of slaggy 

 or ropy lava in 1898, a moving mass 

 of scoriaceous lava and the growing 

 cone in the same year, while a photo- 

 graph of the latter, taken at night by 

 the light of the incandescent lava 

 streams themselves, excited much 

 interest, and it was followed by 

 another showing the same cone in 

 1906 covered, and its surface obscured, 

 by a thick coating of ash from the 

 last eruption. 



In the Strombolian type of eruption 

 the explosion takes place from more 

 or less liquid lava, of which portions 

 are hurled into the air, and by their 

 rotation often assume pointed or even 

 globular forms, which are permanently 

 preserved by the solidification of the 

 mass while in the air. In the 

 Vulcanian type, the materials, which 

 are affected by the explosion, being 

 already solidified, the ejecla are chiefly fragmentary, vary- 

 ing from dust of microscopic fineness to " bread crust 

 bombs '" weighing several tons. The hitter owe their name 



he foreground. 



and their various constituent parts, their relations to one 

 another and to adjacent rocks and to other objects; how 

 they got to their present position and what effects they 

 produce ; in other words, the physio- 

 graphy or physical geography of vol- 

 canoes. -As this subject is itself too 

 large, I propose to take up the lat' 

 eruption of Vesuvius, alluding to othii 

 volcanoes and their eruptions only b\ 



way of illustration, comparison, ir '~c^ ■ •. • .- 



contrast. 



The south-east side of the Bay ol 

 Naples, which consists of the Sorrento 

 Peninsula and the island of Capri, i^ 

 a branch of the Apennine Chain; ii 

 consists largely of .Apennine limestonf 

 of Cretaceous age, and is not volcani( . 

 The north-west side of the Bay, on 

 the contrary, is almost entirely vol- 

 canic. Thus the island of Ischia i^ 

 subject to severe earthquakes, and 

 contains Monte Epomeo, which ha^ 

 been twice in active eruption in 

 historic times; the islands of Procid.i 

 and Nisida contain craters ; tVf 

 Phlegrsean Fields consist of numerous 

 cones and craters, one of which. 

 Monte Nuovo, was formed as recenth 

 as the si.xteenth century. Moreover. 

 Naples itself is built on volcanii 

 strata. The whole district is subjeci 

 to changes of level. 



Coming now to Vesuvius, it is .1 

 matter of common knowledge that ii 

 consists of twin mountains, of which 

 one, Somma, is part of an old 



crater ring of gigantic size which no longer forms part of , to the characteristic crackings on the surface caused by 

 the working cone, which it partly surrounds, and from ] the contraction of the crust when suddenly projected into 



:^i;^ 



Tying away Cooks Railway. April, 1906, 



which 



1 Substance of 

 York on Friday t 



eparaied by a great valley, called in different 



discourse delivered before the British As 

 ning, August 3, by Dr. Tempest Anderson. 



NO. IQ25, VOL. 74] 



the cold air, perhaps aided in some cases by the expansion 

 of gases set free in the interior owing to the sudden re- 

 duction of pressure. 



