MICHIGAN ACADEMY OF SCIENCE. 53 



off the elevated areas and depositing their waste upon the seashore in the 

 neighborhood of the land. Such changes inevitably involve a new distribu- 

 tion of the load pressing upon the rocks within the zone of flow, and no argu- 

 ment is needed to show that somewhere beneath the surface a new distril)u- 

 tion of material by lateral movements must take place in order, in part at 

 least, to bring about adjustment to the. new and ever changing conditions. 

 At any moment places can be found where a strong tendency exists for the 

 withdrawal of some of the material and the supply of a corresponding need 

 elsewhere. A tendency towards withdrawal of material is at the surface 

 above a tendency towards subsidence or settlement; whereas a tendency 

 toward elevation must exist over those districts toward which the material 

 tends to be transferred. 



>50 long as the transfer is delayed, the downward acting forces within the 

 region about to be depressed are met and balanced by equal upward forces 

 due to the rigidity of the rock ]:)risms in the zone of fracture under the strong 

 compression which results from earth contraction. Within the areas about 

 to be elevated, the upward tending forces are similarly met, and the blocks 

 are held rigidly as though between the jaws of a vice. We may illustrate 

 these conditions by a very simple experiment. Within a long narrow tank, of 

 which one side is formed of strong plate glass, is fitted loosely near one of the 

 ends a wall which is hinged upon the bottom. An iron rod of length suffi- 

 cient to project beyond the wall when fastened in a horizontal position to 

 the opposite end of the tank, is supplied Avith screw-thread and nut so as to be 

 used as a vice in compressing any bodies within the tank and large enough 

 to occupy most of its area. The tank is partially filled with water, upon which 

 are supported rectangular prismatic wooden blocks which loosely fill the 

 space. Through varying the height of the blocks the}^ are made to float and 

 project by different amounts above the water surface. When the vice has 

 been tightened they may, however, be made to retain other than their natural 

 positions of flotation. 



If, now, a board of such size as to fit loosely over the blocks in the tank be 

 allowed to rest its weight lightly upon them, all may be brought to the same 

 surface level, and if the vice be properly adjusted, may be retained in that 

 }:>osition when the Ijoard is removed. (See Fig. 1.) Tightly compressed in 

 the vice, the bridge of blocks is held in place against forces tending to ele- 

 vate it throughout those areas where blocks have greatest depth, and to 

 depress it where the block depth is least. If the compression upon the 

 blocks be now gradually removed, a point will at last be reached when the 

 rigidity of the bridge of blocks regarded as a beam is insufficient to hold it 

 in its present attitude, and adjustment will take place. (See Fig. 2.) This 

 adjustment occurs by certain blocks being forced upward and others down- 

 ward, and when a tr'ansfer of water goes on from beneath the latter to 

 the former. Such adjustments of level among the blocks in the l^ridge cor- 

 respond to adjustments of crust blocks at the time of earthcjuakes, and to 

 the formation of earthquake faults upon the block margins. 



The water which ascends between the blocks, owing to the fact that they 

 are not perfectly fitted to each other, represents in the experiment the under- 

 ground water which fills the fissures in all rock masses from very moderate 

 depths down to the zone of flow. As we have seen, the underground water 

 flow is thoroughly deranged at the time of earthquakes, so that within one 

 portion of the affected district the springs flow with unusual volume and 

 bring large quantities of sand and mud to the surface, and in other parts of 

 the same district the water of ponds and swamps is as suddenly sucked down 



