Fig. 384.
Fig. 385.
Fig. 384.—The completion of the hexagonal columns.
Fig. 385.—Diagram to illustrate the development of five-sided columns.
Volcanic cones are familiar structures built up about the vents of active volcanoes, and will be discussed under vulcanism.
STRUCTURAL FEATURES ARISING FROM DISTURBANCE.
Inclination and folding of strata.—The original attitude of beds, whether formed by water or by lava-flows, is normally horizontal, or nearly so. Both kinds of deposits, however, occasionally take place on considerable slopes. Modifications of the original attitude result from earth movements, and the measurement of these modifications is an important feature of field study. It is recorded in terms of dip and strike. The dip is the inclination of the beds referred to a horizontal plane, as illustrated in [Fig. 386], and is usually measured by a clinometer, the principle of which is shown in [Fig. 387]. In measuring the dip, the maximum angle is always taken. In [Fig. 386], for example, the angle would be less if the direction were either to the right or left of that indicated by the arrow. The direction as well as the amount of the dip is always to be noted. This must be determined by the compass, to which the clinometer may be conveniently attached. Dip 40°, S. 20° W. gives the full record of the position of the bed of rock under consideration. The strike is the direction of the horizontal edge of dipping beds, or more generally, the direction of a horizontal line on the surface of the beds. This is illustrated in [Fig. 386]. Since the strike is always at right angles to the dip, the strike need not be recorded if the direction of the dip is. Thus dip 40°, S. 20° W. is the same as dip 40°, strike N. 70° W.
Fig. 386.—Diagram illustrating dip and strike. (Geikie.)
