The main lava stream descended the first steep slopes with the velocity of a mile in twenty-five minutes, about the strolling speed of a pedestrian, but this rate was gradually reduced as the stream advanced farther from the mouth. Taking advantage of each depression of the surface, the black stream advanced slowly but relentlessly toward the cities at the southwest base of the mountain. With a motion not unlike that of a heap of coal falling over itself down a slope, the block lava advances without burning the objects in its path (Fig. 137). [Illustration: FIG. 137.—The main lava stream of 1906 advancing upon the village of Boscotrecase.] [Illustration: FIG. 138.—An Italian pine snapped off by the lava and carried forward upon its surface as a passenger (after Haug).] The beautiful pines are merely charred where snapped off and are carried forward upon the surface of the stream (Fig. 138). When a real obstruction, such as a bridge or a villa, is encountered, the stream is at first halted, but the rear crowding upon the van, unless a passage is found at the side, the lava front rises higher and higher until by its weight the obstruction is forced to give way (Figs. 139 and 140). [Illustration: FIG. 139.—Lava front both pushing over and running around a wall which lies athwart its course (after Johnston-Lavis).] [Illustration: FIG. 140.—One of the villas in Boscotrecase which was ruined by the Vesuvian lava flow of 1906. The fragments of masonry from the ruined walls traveled upon the lava current, where they sometimes became incased in lava.] =The sequence of events within the chimney.=—The thorough study of this Vesuvian eruption has placed us in a position to infer with some confidence in our conclusions the sequence of events within the chimney and crater of the volcano, both before and during the eruption. Anticipating some conclusions derived from the observed dissection of volcanoes, which will be discussed below, it may be stated that what might be termed the core of the composite cone—the chimney—is a more or less cylindrical plug of cooled lava which during the active period of the vent has an interior bore of probably variable caliber. This plug in its lower section appears in solid black in all the diagrams of Fig. 141. During the cone-building period (Fig. 141 _a_ and _b_) the plug is obviously built upward along with the cone, for lava often flows out at a level a few hundred feet only below the crater rim. By what process this chimney building goes on is not well understood, though some light is thrown upon it by the post-eruption stage of Mont Pelé in 1902-1903 (see below). [Illustration: