CHAPTER II. 1 

 MAGNETIC OBSERVATIONS IN OEOLOGICAL MAPPING. 



SECTION I. INTRODUCTION. 



As has been said already, the area in which our work was done is 

 largely drift covered, to somewhat varying but usually considerable depths; 

 the mantle on the whole is so evenly spread that outcrops of any rocks 

 except those belonging to the Archean are in many sections few and scat- 

 tered and sometimes are almost entirely lacking over whole townships. 



Under these circumstances, and since also the pre-Cambrian rock 

 structure is complex, even a general outlining of the old formations would 

 be. impossible by the usual geological methods, and if we were restricted to 

 these there would be no alternative but to map most of the territory as 

 Pleistocene. It happens, however, that the Algonkian rocks of Michigan 

 contain a large amount of magnetite, which is known from observation in 

 the developed ranges to be characteristic of certain geological formations. 

 It undoubtedly occurs in more or less amount in all the sedimentary rocks 

 and is also present, sometimes in considerable quantities, in rocks that are 

 not sedimentary, as, for example, around the margins of the old intrusive 

 diorite bosses. But generally speaking, its occurrence in large quantities is 

 confined so closely to definite geological formations, in which it is found in 

 characteristic association with certain other minerals, or to horizons within 

 those formations, that it can be guardedly used in identifying them, and in 

 tracing them from localities where they outcrop through areas in which they 

 are buried. This use is not only justified, from an empirical standpoint, by 

 the presumption in favor of analogies to which no exceptions are known, but 

 it has a rational basis, in the \iew of the late Professor Irving,^ which is 



' This chapter is abridged from a paper of the same title presented at the Colorado meeting of 

 the American Institute of Mining Engineers in September, 1896. 



2 Classificatioi of early Cambrian and pre-Cambriau formations, byR. D. Irving: Seventh Ann. 

 Kept. U. S. Geol. Survey, 1888, pp. 451-452. 

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