DEMONSTRATION MATERIAL IN GEOLOGY 73 



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contact lines at different periods in the pool's history. The surface of 

 the region is represented by a sheet of transparent celluloid, on which is 

 drawn the map of the region; wooden pegs represent the wells drilled. 



It is hoped that arrangements can be made by which this model can 

 be offered for sale. 



1). The model of the Hewitt field constructed by the Bureau of Mines, 

 and described in Bulletin 195 of the Bureau, could probably be dupli- 

 cated for about $50. At least one such model could be used to advantage 

 in elementary classes. 



c. A peg model of a salt dome on the coast is being constructed by the 



Figure 14. — Another View of the Model shown in Figure IS, top removed 



Texas Geological Society, but whether it can be sold when completed is 

 not yet known. This would probably be of value for advanced classes. 



12. Prof. J. Austin Bancroft calls attention to a type of model, a 

 modification of the "glass-plate" model, which he devised while working 

 out the underground geology of a large mine. He secured large sheets 

 of thick, transparent celluloid and used diluted India ink of different 

 colors in outlining the geology of the various mine levels on these sheets. 

 He had a wooden frame constructed whereon he could mount these geo- 

 logically colored celluloid sheets in a manner similar to the ordinary 

 glass model of a mine. Then he had made some dowel sticks, l/16th and 

 ]/8th of an inch in diameter, colored to show the geological formation 

 passed through by each diamond-drill hole, and in order to give rigidity 



