The form which the materials have, when ejected from volcanoes, depends mainly upon the degree of liquidity of the lavas at the volcanic foci. If the liquidity is very perfect, the aqueous vapor will readily rise through the lava. The steam thus separated will drive before it whatever rocks, or previous lavas, may obstruct it. In their progress they would be reduced to sand and powder, and ejected as _volcanic cinders_. (Fig. 79.) If the lava possess considerable viscidity, the aqueous vapor will separate with more difficulty, and the lava and vapor will ascend the channel together. Large bubbles of vapor will, however, collect with more or less of frequency; and, as they rise through the lava, will drive forward a portion of it, and cause the overflow to take place by pulsations. As the bubbles reach the surface, their bursting causes the loud reports, which are compared to the discharge of heavy artillery. With each explosion some of the lava will be projected violently into the air, and, cooling, will fall to the surface as scoriæ,--or, if the lava be highly vitreous, it will be drawn out into fibres, and descend as volcanic glass. III. _Geological Phenomena referable to Volcanic Action._ Volcanic agency has probably never been less than it is now, and we ought therefore to find its effects very general and important. 1. The most obvious of these effects are the _fractures_ with which the crust of the earth is everywhere intersected. The uplifting force upon which all volcanic phenomena depend would necessarily fracture the crust, and the wave-like motion resulting from the fracture would cause numerous secondary fractures, having a parallel direction. They are often of such extent, during earthquakes, as to endanger life. During the great earthquake at Lisbon, in 1755, a fracture opened of sufficient width to swallow up the quay, and several thousands of persons who had fled there for safety. The chasm remained permanently open to the depth of six hundred feet. The earthquakes with which the valley of the Mississippi was visited in 1811 so often fissured the surface, that the inhabitants protected themselves by clinging to the trunks of trees, which they felled transversely to the direction of the fissures. [Illustration: Fig. 80.]