224 THE ERUPTION OF VESUVIUS IN APRIL, 1906. 



for example, being considered characteristic accompaniments of erup- 

 tions of acid magna. At Mont Pelee, however, the same magma 

 sufficiently acid to form rocks rich in quartz, although not changing 

 in composition with time, has been emitted as very liquid flows of 

 considerable length, as masses of viscous lava accumulated around 

 the orifice and accompanied with violent explosions, and finally as 

 pumiceous material resulting from eruptions entirely explosive. 

 The fact must be emphasized, therefore, that the form of eruption 

 is determined not only by the chemical composition of the magma, 

 but also by its physical condition, and by its fluidity or viscosity at the 

 moment of eruption. 



It is doubtless true, that, as the fusibility of a magma bears a rela- 

 tion to the chemical composition, a very basic magma tends to reach 

 the surface in a more fluid condition than a very acid magma, but 

 there are many conditions — velocity of emission, mass of material 

 emitted, temperature, and abundance of volatile products, notably 

 water vapor — which can modify the fundamental tendency and cause 

 a volcano with very fusible lava to behave as a volcano with viscous 

 lava, or vice versa. Former eruptions of Vesuvius, as well as that of 

 1906, furnish numerous arguments in support of this proposition. 



The most fluid magma known is the basaltic lava of Kilauea and 

 Mauna Loa. It is a thin, opaque liquid of great fluidity. The 

 emission is accompanied by no violent explosions, and only by light 

 vapors. This type has been called the Hawaiian type. 



The fluidit}^ of the basaltic magma of Stromboli is still great at 

 the moment of eruption, although less than in the preceding case. 

 The discharge of gases causes violent explosions, which throw into 

 space fragments of the doughy magma, some of which fall upon the 

 edges of the crater to flatten there, while other portions shape them- 

 selves in the air and fall as scoria, either in blocks or in fine dust. 

 Rock fragments, already consolidated, caught in the magma, form 

 elongated bombs. The ejected material shows its incandescence even 

 in daylight, and at night forms admirable fireworks. Water vapor 

 is often hardly apparent; when it is visible it forms white thin 

 clouds. This type of explosion I designate with Mercalli the Strom- 

 holian type. 



An altogether different type was realized in 1888-89 at Vulcan o 

 for which Mercalli has proposed the name Vulcanian. It is produced 

 when at the moment of explosion the magma is very viscous or wholly 

 consolidated; in the first case, the bombs have the bread-crust struc- 

 ture with a pumiceous center and a glassy exterior; in the second 

 case, they are formed of angular blocks which on cooling become 

 fissured by contraction. The clouds accompanying the Vulcanian 

 explosions are very dense, opaque, gray or black, with outlines ex- 

 tremely well defined, and lightning is frequent among them. The 



