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Page:Popular Science Monthly Volume 18.djvu/798

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THE POPULAR SCIENCE MONTHLY.

direction slanting away from the axis, until the beds reach the maximum angle of repose of the rock-fragments in question as the beds (D D) on diagram. Thus we have constructed a conical mass in the center of which is the volcanic chimney (B), and, dipping away on all sides at angles, varying generally between 20° and 45°, we find the strata composing the cone (D, E). This arrangement is often called

PSM V18 D798 Diagrammatic view of a volcanic cone.jpg
Diagrammatic bird's-eye view of a volcanic cone.—The upper part is supposed to be removed by an horizontal section and one half of the remaining base by another longitudinal one. A, vent; B, chimney; C, basement rock, compressed downward at C' and upward at C"; D, ash-beds; E, lava streams, one of which, E' is seen to have run down the slopes, G, of the cone, and spread over plain F.

periclinal. The funnel, or chimney, which has been mentioned as occupying the center, has the form of an inverted cone, the inclination of its sides and its diameter necessarily being proportional to the volume and force of the escape of vapor, and also to the nature, form, and size of the surrounding fragments, forming the growing cone, which have already been ejected. The upper, or basin part, is technically called the crater. The vapor only may have made its appear-