THE ERUPTION OF VESUVIUS IN APRIL, 1906. 247 



From this time the eruption may develop in either of two ways. 

 In the most frequent case, which may be characterized with Mercalli 

 as the type of 1895, the flow is tranquil and prolonged for several 

 months. At the beginning the crater deepens and Vulcanian explo- 

 sions take place, but as the lava flows away Strombolian or mixed 

 explosions succeed, increasing in intensity as the flow ceases. The 

 eruptions of October, 1751; April, 176G; August, 1834; May, 1859; 

 December, 1881; June, 1891; July, 1895; and August, 1903, are 

 examples. 



In the second type, that of 1872, the outflow is violent and rapid 

 and lasts only a day or a few hours. The maximum activity immedi- 

 ately precedes or is contemporaneous with the outflow^ of lava. 

 Explosive phenomena take place at times also at the lateral fissure. 

 The crater enlarges by the explosions, the summit sinks, and at the 

 end of the eruption the mountain, Avith diminished height, has a wide, 

 deep crater. The eruptions of 1031, 1737, 17(57, 1779, 1822, 1839, 1850, 

 1855, 1868, and 1872 are examples. 



The eccentric eruptions (type of 17G0), which are the rule on Etna, 

 are rare on Vesuvius (17G0, 1794, 1861). The lava then comes from 

 adventitious cones that form on the southeast slope of Monte Somma 

 at altitudes from 500 to 300 meters. 



The eruptions of the types of 1872 and 1760 always close a period 

 of activity and are invariably followed by a period of repose. The 

 recent eruption belongs to the type of 1872 and closes the period of 

 activity almost continuous since 1875. It presents the dominant 

 characteristics of the type — a rapid and short flow of lava from the 

 flanks; violent Vulcanian explosions, destroying the summit and 

 forming a caldera ; and complete cessation of activity. No new 

 phenomena have been noted in the course of this eruption; the flow 

 is comparable to that of 1872, but the intensity of explosion has been 

 greater and more like those of 1779 and 1822. 



The results of the study of the dry avalanches, the mechanism of 

 their production, their action upon the topography of the cone, the 

 breccias that they have formed, and the facts regarding metamor- 

 phism furnished by the blocks ejected by the Vulcanian explosions, 

 constitute, in my opinion, the principal acquisitions that science owes 

 to the eruption. 



It is particularly suggestive to see opposed characteristics reunited 

 in the same eruption and realized successively with equal intensity. 

 Vesuvius has acted at first under the form common to basic magmas 

 and tken under that regarded as characteristic of acid magmas. It 

 has furnished Strombolian ejections of incandescent Inva and long 

 and rapid flows like those of basalt, and finally, by the Vulcanian 



