CHARACTER AXD KIXDS OF GEOLOGIC AIAPS 1115 



right angles to the original shoreline. The two quadrangles mapped 

 have only formations b, c, and d (called h'^, c\ d^) in common. 

 Formation &^ of the western quadrangle is, however, more nearly- 

 equivalent to the upper part of c, as it appears in the eastern quad- 

 rangle. The bed c^ of the western quadrangle is only a part — the 

 lower — of bed c as it appears in the eastern quadrangle, while d^ 

 and d are almost exact equivalents. In the eastern quadrangle occur 

 the formations a and e, which are not found in the western 

 one, while the latter shows formations /, g, h, i, m and y, not found 

 in the eastern quadrangle. In the eastern quadrangle, furthermore, 

 the formations differ on opposite sides of the valley, h being tliin 

 on the eastern and thick on the western side, while c and d of the 

 western side are represented by e on the eastern side. 



Types of Geological Maps. 



Formation and System Maps. Two kinds of geological maps 

 are in vogue in most countries. These are the formation map and 

 the system map. The first takes account of the geological forma- 

 tions, and is illustrated by the folio sheets of the Geological Atlas 

 of the United States, already referred to. In these each formation 

 is given a distinct color, or pattern, all the formations of a system 

 generally being grouped together under a similar tint, such as pink, 

 brown, etc. For the production of such maps, large scale base- 

 maps are needed, those used by the United States Geological Sur- 

 vey being mostly on the scale of i to 62,500, or approximately i 

 inch to a mile. As the scale of the map is increased, smaller units 

 can be mapped, and structural details not representable on the 

 smaller scale map may be introduced. The system map aims to 

 represent in distinctive colors only the geologic systems, each of 

 which receives a distinct color pattern. The new geological map 

 of North America, issued by the United States Geological Survey, 

 may be taken as an example of this type. Here eighteen distinct 

 color shades are used to represent the systems from the Cambric 

 to the Quaternary, though for convenience the boundary in a few 

 cases is not drawn precisely at the dividing line between the 

 systems. 



The International Geological Map of Europe is also a system 

 map, though the attempt has there been made to differentiate by 

 distinctive shades the lower, middle and upper portions of some of 

 the systems {e. g., Triassic, Jurassic, etc.). 



Intermediate Maps. Maps intermediate between the system and 



