EARTH RESISTIVITY AND GEOLOGICAL STRUCTURE 169 



instance, at the test sites included in the second group of the summary 

 the upper part of the structure consists of Ordovician rocks. These 

 are underlain by Cambrian strata which, in turn, lie on the pre- 

 Cambrian base. The question arises whether the results were in- 

 fluenced by the Ordovician strata alone, by both the Ordovician and 

 Cambrian, or by the Ordovician, Cambrian, and pre-Cambrian. 

 Calculations have been made which indicate that probably only the 

 Ordovician and Cambrian strata were involved to any important 

 extent. The tests in the other groups have been treated in a similar 

 manner. 



In the cases considered above the measurements were apparently 

 influenced largely by strata of a single period or of two or more periods 

 of about the same age. The problem is not always as simple as this. 

 For instance, in some areas combinations of very old and very young 

 strata occur. Areas in which the oldest rocks, the pre-Cambrian, lie 

 directly under comparatively thin sediments of the latest periods — 

 the Quaternary, Tertiary, and Cretaceous — are not uncommon. The 

 efi"ective resistivities shown by the tests in such areas range between 

 very wide limits and the tabulation should not be taken as indicative 

 of the values which may prevail under such conditions. 



The eff"ects of soils, glacial drift, alluvial deposits along the courses of 

 streams, and other surface materials may also in some instances be 

 such as to result in effective resistivities differing widely from those 

 that would be indicated by the tabulation. The effect of local alluvial 

 deposits, where they overlie the older rock strata, is to lower very 

 materially the effective resistivity that would be expected were the 

 deposits not present. 



Another limitation which must be considered is concerned with the 

 presence of rocks formed by volcanic action. Apparently such rocks 

 usually have a high resistivity and where they occur in a comparatively 

 young structure, the effective resistivity may be much higher than the 

 tabulation would indicate. 



The test results indicate also that the effective resistivities of 

 structures of given periods within certain large geographical regions 

 are markedly different from those of structures of the same periods in 

 other large regions. Within any one of such regions, excluding areas 

 where igneous and highly metamorphosed sedimentary rocks are in- 

 volved, the effective resistivities of structures of the same period are 

 encompassed within a comparatively narrow band. 



To facilitate the use of the correlation data, the different areas within 

 which tests have been made have been divided into groups in accord- 

 ance with the geological periods of the uppermost strata within these 



