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BELL SYSTEM TECHNICAL JOURNAL 



areas and maps have been prepared which show the boundaries of 

 these areas and the results of the tests within them. Two such maps 

 are shown in Figs. 1 and 2. These maps show the geological periods of 

 the upper strata as they would appear were the overlying mantle of 

 soil, glacial drift, local alluvial deposits and such removed. Any one 



Fig. 1 — Areal geology and effective resistivities — pre-Cambrian areas. 



"test," as the designation is employed here, may include measurements 

 of ground-return mutual impedances for a number of different con- 

 ductors or sections of line and the test results for these different 

 conductors or sections may indicate quite a wide range of resistivities — 

 often 2 or 3 to 1 and occasionally 10 or more to 1, depending upon the 



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