THE ERUPTION OF VESUVIUS IN APRIL, 1906. 225 



solid matter transported by them, whatever its dimensions, is formed 

 of fragments or dust of rock entirely consolidated. Perfect examples 

 of Vulcanian clouds have been furnished by Vulcano (trachitic), 

 Mount Pelee (andesitic), and by the recent eruption (April, 1906) 

 of Vesuvius (leucitic). 



This idea of Strombolian and Vulcanian explosions, although not 

 formulated till the past few 3'ears, has been long recognized. Fouque 

 at the time of the eruption of Etna, in 1865, noted that some craters 

 gave forth liquid lava with almost colorless vapors, whereas others 

 ejected only solid materials and a thick mixture of water vapor and 

 dust. 



It is necessary, finally, to consider a variety of the Vulcanian type, 

 which I will call Peleecm. It differs essentially in that the propor- 

 tion of solid materials carried away by the cloud is much greater, 

 so that, instead of ascending vertically, as in the preceding case, it 

 descends upon the flank of the volcano and flows or rolls upon the 

 surface of the ground with accelerated velocity. Sometimes, as in 

 the eruption of St. Vincent (1902) and the small eruptions of Mont 

 Pelee, its motion is essentially the result of gravity alone; at other 

 times, as in the great paroxysms of Mont Pelee, the cloud has been 

 directed by an initial thrust and by gravity working in the same 

 direction. A Peleean cloud is also animated by an ascensional move- 

 ment due to the expansion of water vapor, but this is of secondary 

 origin, and its direction is not the same as that of the initial thrust. 

 The cloud ascends vertically as it rolls downward upon the surface of 

 the slope. 



I employ the term Peleean cloud as more general than that of 

 burning cloud (nuee ardente), because similar clouds are conceivable 

 at temperatures inferior to that characterizing the eruptions of the 

 Antilles, although the high temperature undoubtedly plays an im- 

 portant part in the mechanism of the cloud and in the stability of 

 the emulsion of solid material in the water vapor that characterizes it. 



The different types of explosions form a continuous series. Fur- 

 ther divisions are unnecessaiy, intermediate types being referred to 

 as mixed explosions. 



SEQUENCE OF PHENOMENA OF THE ERUPTION. 



The recent eruption of Vesuvius (April, 1906) had been a long time 

 in preparation. In April, 1905, the explosive activity of the volcano 

 increased. At the bottom of the crater, at a depth of 80 meters, a 

 small cone was found, which, by successive Strombolian explosions, 

 had grown by the middle of May to a height of 15 meters above the 

 edges of the old crater. 



SM 1906 — —15 



