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else of memory; the latter demauds that we not only accumulate facts, 

 but that we seek a rational explanation of the facts observed. And just 

 so far as we can see the relationships of the factors concerned in a geo- 

 graphical problem, and the role each has played in producing the ob- 

 served results, to that degree have we gained real and useful scientific 

 knowledge. 



It was with this fundamental principle in mind that I have set about 

 to prepare some geographical helps to attain this end. Any piece of 

 apparatus such as a geological model, or map which properly expresses 

 an evident relation between the geology or rock structure and the to- 

 pography provides good material from which may be gained genuine geo- 

 graphical knowledge. Such material, however, is often in poor form and 

 shape for laboratory use, and more often quite useless for lecture pur- 

 poses, the scale being too small, or facts not well expressed. The ma- 

 terial I describe below is intended primarily for use in lecture work. 

 It consists of a lantern slide of a model representing a type of land form, 

 and showing at once the relief of the land as well as the rock structure 

 in two cross sections. With the picture of a model which brings out 

 clearly the relations of structure to topography, and all the larger features 

 of adjustment of drainage to structure, the lecturer can actually show 

 up the facts as well as the arguments leading to his interpretation of the 

 actual history of the land form discussed. Such details as could not be 

 shown on ordinary maps may be clearly depicted by this method of illus- 

 tration. 



The data used in the construction of the illustrated model were 

 gathered from the Geological Atlas sheets published by the United States 

 Geological Survey. The area selected is that covered by the Harpers 

 Ferry sheet. From the data therein contained, a model was constructed 

 on the scale of one inch to the mile, vertical scale one inch to sixteen 

 hundred feet. 



The method used in the construction of the base may be aptly termed 

 the contour method. The course of procedure was as follows: The 

 topographic sheet was first enlarged to the desired scale. In the case 

 of Harpers Feny it was enlarged from two miles to the inch to one 

 mile to the inch. The culture in addition to the topography was also 

 transferred to the enlarged sheet and the whole traced on tracing cloth. 

 The next step was to determine the vertical scale which would give the 

 most expressive and yet close approach to the natural appearance of 



