CHAPTER XVI 



GEOLOGICAL MAPS 



THE surface of landscape is formed by soil resting on subsoil, 

 but under the latter, at varying distances, more or less solid 

 rock will always be found. Its situation and the extent of any 

 well-marked mass of it is called the outcrop of the rock. A 

 geological map is designed to show the outcrops of the various 

 rock masses as they would be seen if soil, subsoil, and a few other 



numuim 



FIG. 49. Geological map of a small coalfield, showing outcrop of a coal- 

 seam. At the side is the section shown by the shaft of the colliery. 



superficial materials were to be removed. These different rocks 

 are expressed by varying tints or shading, and numerous con- 

 ventional signs are employed to enable the reader of the map to 

 understand the relations of the rocks to one another. 



For instance, a strong black line on the map (Fig. 49) is intended 

 to indicate the outcrop of a seam of coal, a stratum or bed 4 or 5 feet 

 thick. This plunges underground on one side or other of the line, 



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