THE ERUPTION OF VESUVIUS IX APRIL, 1906. 231 



days following- the chief paroxysm we passed hours watching these 

 clouds. Starting from the crater they remained motionless on its 

 edges, affecting at times a slight descending motion, wdiich, however, 

 was never completed. It seemed that in these cases a 'slight increase 

 in density would have determined the formation of the Peleean 

 clouds. 



The Vulcanian explosions have been not only one of the most 

 imposing spectacles of nature, but have had far-reaching results. 

 They have formed a deep caldera; they have accumulated upon its 

 edges an enormous mass of solid material of all dimensions, at the 

 expense of which the dry avalanches have been formed ; they have 

 caused the disaster at Ottajano and San Giuseppe, and finally have 

 sent into space nuich fine dust, which has covered the volcano with a 

 thick bed, caused injury to agriculture throughout the region, and 

 been transported by the wind to great distances from Vesuvius. 



Formation of a caldera. — At the beginning of April a small cone 

 had been constructed in the old crater, extending several meters above 

 its summit, with an altitude of 1,335 meters above sea level. The 

 Vulcanian explosions of April 4 destroyed this small ephemeral 

 cone and dismantled the summit of the old cone. The paroxysm of 

 April 8 and the following explosions produced the present crater. 

 The operation w^as complex and embraced the following stages : First, 

 the complete evacuation of the new magma, filling the central canal 

 by the Strombolian ejections and especially by the formation of lat- 

 eral flows; second, the shattering of the walls and enlargement; 

 third, a sinking which carried with it the whole summit of the moun- 

 tain; fourth, the ejection of the greatest part of the crumbling 

 material. 



One comprehends easih^ why the explosions were of the Vulcanian 

 type. The interruption of free communication with the exterior 

 brought about such a condition that the explosions were made in a 

 solid medium, like that of very viscous or extremely consolidated 

 lava, though in this case the mass to be raised was not essentially 

 new magma but old debris. The main features of the crater -^x-ere 

 acquired at the end of the night of April 8, during the course 

 of which the principal sinking of the summit took place, but the 

 Vulcanian explosions continued much longer and ended by throwing 

 out of the new cavity the material that partially obstructed it. 



The section of the new crater is almost circular, with dimensions 

 of 640 by 650 meters. The depth appears to be at least 300 meters. 

 The walls are almost vertical, except near the surface, where they 

 form a steep talus, and near the bottom, where they terminate in a 

 funnel, the bottom of which is partly hidden by fumaroles. The 

 crest is shattered, irregular, and sharp-edged. The highest side is 

 on the northwest ; the lowest is a deep notch TT meters lower on the 



