DEMONSTRATION MATERIAL IN GEOLOGY 61 



fixed imaginary plane. Let this map be placed over the necessary horizontal 

 parallel rollers armed with projections or cams so arranged that when the 

 rollers are set in motion in a given direction by a gear connecting them all, 

 the parts of the map which were land in a given geological period will rise 

 above the plane mentioned and parts which were water will sink below that 

 plane. To complete the analogy, water may be introduced to the level of the 

 plane assumed, and the design may be so arranged as to flood the submerged 

 parts of the continent and to let the water flow away as elevation comes on. 

 By a proper adjustment of the cams on the rollers, it seems possible to repro- 

 duce before the eye of the student the changes which are represented at their 

 culmination on the so-called paleogeographic maps. The mechanism can be 

 stopped, of course, at any stage, representing, say, the assumed distribution of 

 land and sea in the Hamilton or in the Cincinnati stage. Corrections in the 

 maps may be carried to the machine by correcting the length of the pegs or 

 cams and changing their position as regards the time when they roll up and 

 elevate or roll down and permit the sinking of the cloth map above them. A 

 crank would suffice to turn the apparatus through the revolutions it was de- 

 signed to execute." 



APPARATUS ALREADY CONSTRUCTED 



Some apparatus has been constructed and is already in general rise and 

 some little known apparatus should be more generally employed. 



1. One of the best models to illustrate normal faulting is shown in 

 figures 1, 2, and 3, but, as far as is known, there is but one copy in this 

 country. The model is 18 inches long and 5 inches wide and consists of 

 a block of wood inclosed between glass plates. The block of wood is cut 

 by two diagonal faults at right angles to each other, and is so constructed 

 that when the lower part of the block on the right is withdrawn a simple 

 normal fault (figure 2) is produced, and when the lower part of the block 

 on the left is removed a second fault (figure 3) forms. When the student 

 has manipulated this model a few times he can readily visualize the ap- 

 pearance and understand the mechanics of "double" faulting which at 

 first seemed to him very complex. 



2. As students have difficulty in understanding slip, dip-slip, strike- 

 slip, net-shift, and drag-dip, a model, such as that shown in figure 4, 

 constructed after the diagram in the report of the Committee on the 

 Nomenclature of Faults, 3 has been found very useful. A convenient size 

 is 6 x 12 x 3 inches. In making this model it is better to use one piece 

 of wood ; otherwise the model is liable to break at the fault-line. 



3. Another very simple piece of apparatus (figures 5, 6, 7) illustrates 

 the effects of both normal and thrust-faults on inclined beds. By turning 

 back the hinged top of the back block, the effect of erosion on a region so 



3 H. F. Reed, W. M. Davis, A. C. Lawson, F. L. Ransome : Report of the Committee 

 on the Nomenclature of Faults. Bull. Geol. Soc. Am., vol. 24, 1913. p. 171. 



