┌───────────────────────────────────────────────────────────────────────┐ │ PLATE 4. │ │ │ │ [Illustration: │ │ │ │ _A._ Experimental tank to illustrate the earth movements which are │ │ manifested in earthquakes. The sections of the earth’s shell are here │ │ represented before adjustment has taken place.] │ │ │ │ [Illustration: _B._ The same apparatus after a sudden adjustment.] │ │ │ │ [Illustration: _C._ Model to illustrate a block displacement in rocks │ │ which are intersected by master joints.] │ └───────────────────────────────────────────────────────────────────────┘ =Derangement of water flow by earth movement.=—The water which supported the blocks in our experiment has represented the more mobile portion of the earth’s substance beneath its outer zone of fracture. The surface water layers in the tank may, however, be considered in a different way, since their behavior is remarkably like that of the water within and upon the earth’s surface during an earth adjustment. At the instant when adjustment takes place in the tank, water frequently spurts upward from the cracks between the sinking end blocks; and if in place of one of the higher center blocks we insert one whose top is below the level of the water in the tank, a “lake” will be formed above it. When the adjustment occurs, this lake is immediately drained by outflow of the water at its bottom along one of the cracks between the blocks (Fig. 76). [Illustration: FIG. 76.—Diagrams to illustrate the draining of lakes during earthquakes.]