6. In a vertical shaft of a coal mine the same bed of coal is pierced twice at different levels because of a fault. Draw a diagram to show whether the fault is normal or a thrust. 7. Copy the diagram in Figure 194, showing how the two ridges may be accounted for by a single resistant stratum dislocated by a fault. Is the fault a STRIKE FAULT, i.e. one running parallel with the strike of the strata, or a DIP FAULT, one running parallel with the direction of the dip? 8. Draw a diagram of the block in Figure 195 as it would appear if dislocated along the plane efg by a normal fault whose throw equals one fourth the height of the block. Is the fault a strike or a dip fault? Draw a second diagram showing the same block after denudation has worn it down below the center of the upthrown side. Note that the outcrop of the coal seam is now deceptively repeated. This exercise may be done in blocks of wood instead of drawings. 9. Draw diagrams showing by dotted lines the conditions both of A and of B, Figure 196, after deformation had given the strata their present attitude. 10. What is the attitude of the strata of this earth block, Figure 197? What has taken place along the plane bef? When did the dislocation occur compared with the folding of the strata? With the erosion of the valleys on the right-hand side of the mountain? With the deposition of the sediments? Do you find any remnants of the original surface baf produced by the dislocation? From the left-hand side of the mountain infer what was the relief of the region before the dislocation. Give the complete history recorded in the diagram from the deposition of the strata to the present. 11. Which is the older fault, in Figure 198, or When did the lava flow occur? How long a time elapsed between the formation of the two faults as measured in the work done in the interval? How long a time since the formation of the later fault? 12. Measure by the scale the thickness lie of the coal-bearing strata outcropping from a to b in Figure 199. On any convenient scale draw a similar section of strata with a dip of 30 degrees outcropping along a horizontal line normal to the strike one thousand feet in length, and measure the thickness of the strata by the scale employed. The thickness may also be calculated by trigonometry. UNCONFORMITY